feat: conversation compacting (manual + automatic)

Implement roadmap item 10: conversation compaction to reclaim context
window without losing the thread.

Wire (0.11.0):
- Add CompactionResult type
- Add ConversationCompactedMessage WS event

Transport-contract (0.15.0):
- Add CompactResponse, CompactThresholdResponse, SetCompactThresholdRequest
- Add ConversationCompactedMessage to WsServerMessage union
- Re-export CompactionResult

Conversation-store:
- replaceHistory: delete all chunks, reset seq, append new messages
- getCompactThreshold / setCompactThreshold (per-conversation setting)
- compactThresholdKey added to keys.ts

Session-orchestrator:
- CompactionService interface + compactionHandle
- conversationCompacted hook descriptor
- createCompactionService: load history, split old/recent, call provider
  to summarize, replaceHistory with [system: summary] + recent N
- Auto-trigger: resolveCompaction lazy dep, fires after turn settles
  (checks threshold, non-blocking)
- Hook declared in manifest contributes.hooks + services

Transport-http:
- POST /conversations/:id/compact (manual trigger)
- GET /conversations/:id/compact-threshold (read setting)
- PUT /conversations/:id/compact-threshold (set setting)

Transport-ws:
- Subscribe to conversationCompacted hook
- Broadcast conversation.compacted WS message

CLI:
- dispatch compact <conversationId> command

FE handoff: frontend-compaction-handoff.md

5 files changed, 119 insertions, 1 deletions

diff --git a/packages/transport-http/src/app.test.ts b/packages/transport-http/src/app.test.ts
index 789efce..51f791f 100644
--- a/packages/transport-http/src/app.test.ts
+++ b/packages/transport-http/src/app.test.ts
@@ -138,6 +138,11 @@ function createFakeConversationStore(
 			return null;
 		},
 		async setConversationStatus() {},
+		async replaceHistory() {},
+		async getCompactThreshold() {
+			return null;
+		},
+		async setCompactThreshold() {},
 	};
 }
 
@@ -859,6 +864,11 @@ describe("GET /conversations/:id", () => {
 					return null;
 				},
 				async setConversationStatus() {},
+				async replaceHistory() {},
+				async getCompactThreshold() {
+					return null;
+				},
+				async setCompactThreshold() {},
 			};
 			const app = createApp({
 				conversationStore: store,
@@ -927,6 +937,11 @@ describe("GET /conversations/:id", () => {
 				return null;
 			},
 			async setConversationStatus() {},
+			async replaceHistory() {},
+			async getCompactThreshold() {
+				return null;
+			},
+			async setCompactThreshold() {},
 		};
 		const app = createApp({
 			conversationStore: store,
@@ -1064,6 +1079,11 @@ describe("GET /conversations/:id/metrics", () => {
 				return null;
 			},
 			async setConversationStatus() {},
+			async replaceHistory() {},
+			async getCompactThreshold() {
+				return null;
+			},
+			async setCompactThreshold() {},
 		};
 		const app = createApp({
 			conversationStore: brokenStore,
@@ -2034,6 +2054,11 @@ describe("PUT /conversations/:id/reasoning-effort", () => {
 				return null;
 			},
 			async setConversationStatus() {},
+			async replaceHistory() {},
+			async getCompactThreshold() {
+				return null;
+			},
+			async setCompactThreshold() {},
 		};
 		const app = createApp({
 			conversationStore: store,
diff --git a/packages/transport-http/src/app.ts b/packages/transport-http/src/app.ts
index e9f56c9..fd78f3e 100644
--- a/packages/transport-http/src/app.ts
+++ b/packages/transport-http/src/app.ts
@@ -1,6 +1,8 @@
 import type { AgentEvent, HostAPI, Logger } from "@dispatch/kernel";
 import type {
 	CloseConversationResponse,
+	CompactResponse,
+	CompactThresholdResponse,
 	ConversationHistoryResponse,
 	ConversationListResponse,
 	ConversationMetricsResponse,
@@ -12,6 +14,7 @@ import type {
 	OpenConversationResponse,
 	QueueResponse,
 	ReasoningEffortResponse,
+	SetCompactThresholdRequest,
 	ThroughputResponse,
 	TitleResponse,
 	WarmResponse,
@@ -36,6 +39,7 @@ import {
 	serializeEventLine,
 } from "./logic.js";
 import {
+	type CompactionService,
 	type ConversationStore,
 	type CredentialStore,
 	conversationOpened,
@@ -52,6 +56,7 @@ export interface CreateServerOptions {
 	readonly orchestrator: SessionOrchestrator;
 	readonly credentialStore: CredentialStore;
 	readonly warmService?: WarmService;
+	readonly compactionService?: CompactionService;
 	readonly lspService?: LspService;
 	/** Optional — defaults to a no-op store (recording disabled, empty reports). */
 	readonly throughputStore?: ThroughputStore;
@@ -683,6 +688,79 @@ export function createApp(opts: CreateServerOptions): Hono {
 		}
 	});
 
+	// ─── Compaction ──────────────────────────────────────────────────────────
+
+	app.post("/conversations/:id/compact", async (c) => {
+		if (opts.compactionService === undefined) {
+			return c.json({ error: "Compaction service not available" }, 503);
+		}
+		const conversationId = c.req.param("id");
+		let body: unknown = {};
+		try {
+			body = await c.req.json();
+		} catch {
+			// No body is fine — use defaults.
+		}
+		const obj = body as Record<string, unknown>;
+		const keepLastN =
+			typeof obj.keepLastN === "number" && Number.isFinite(obj.keepLastN) && obj.keepLastN > 0
+				? Math.floor(obj.keepLastN)
+				: undefined;
+		const modelName = typeof obj.modelName === "string" ? obj.modelName : undefined;
+
+		log.info("conversations: compact request", { conversationId });
+
+		const result = await opts.compactionService.compact(conversationId, {
+			...(keepLastN !== undefined ? { keepLastN } : {}),
+			...(modelName !== undefined ? { modelName } : {}),
+		});
+
+		if ("error" in result) {
+			log.warn("conversations: compact returned error", {
+				conversationId,
+				error: result.error,
+			});
+			return c.json({ error: result.error }, 409);
+		}
+
+		const response: CompactResponse = {
+			conversationId,
+			messagesSummarized: result.messagesSummarized,
+			messagesKept: result.messagesKept,
+		};
+		return c.json(response, 200);
+	});
+
+	app.get("/conversations/:id/compact-threshold", async (c) => {
+		const conversationId = c.req.param("id");
+		const threshold = (await opts.conversationStore.getCompactThreshold(conversationId)) ?? 0;
+		const response: CompactThresholdResponse = { conversationId, threshold };
+		return c.json(response, 200);
+	});
+
+	app.put("/conversations/:id/compact-threshold", async (c) => {
+		const conversationId = c.req.param("id");
+		let body: unknown;
+		try {
+			body = await c.req.json();
+		} catch {
+			return c.json({ error: "Invalid JSON body" }, 400);
+		}
+		const parsed = body as SetCompactThresholdRequest;
+		if (
+			typeof parsed.threshold !== "number" ||
+			!Number.isFinite(parsed.threshold) ||
+			parsed.threshold < 0
+		) {
+			return c.json({ error: "threshold must be a non-negative number" }, 400);
+		}
+		const threshold = Math.floor(parsed.threshold);
+		await opts.conversationStore.setCompactThreshold(conversationId, threshold);
+		log.info("conversations: compact-threshold set", { conversationId, threshold });
+		const response: CompactThresholdResponse = { conversationId, threshold };
+		return c.json(response, 200);
+	});
+
 	// ─── Static frontend serving (catch-all, API routes take precedence) ──────
 	if (opts.webDir !== undefined) {
 		const webDir = opts.webDir;
diff --git a/packages/transport-http/src/extension.ts b/packages/transport-http/src/extension.ts
index 3555f8e..351dd4a 100644
--- a/packages/transport-http/src/extension.ts
+++ b/packages/transport-http/src/extension.ts
@@ -2,6 +2,7 @@ import type { Extension, HostAPI, Manifest } from "@dispatch/kernel";
 import { createApp } from "./app.js";
 import {
 	cacheWarmHandle,
+	compactionHandle,
 	conversationStoreHandle,
 	credentialStoreHandle,
 	lspServiceHandle,
@@ -30,6 +31,8 @@ export const manifest: Manifest = {
 			"/conversations",
 			"/conversations/:id",
 			"/conversations/:id/close",
+			"/conversations/:id/compact",
+			"/conversations/:id/compact-threshold",
 			"/conversations/:id/cwd",
 			"/conversations/:id/last",
 			"/conversations/:id/lsp",
@@ -61,6 +64,7 @@ export function createTransportHttpExtension(): Extension & {
 			const credentialStore = host.getService(credentialStoreHandle);
 			const throughputStore = host.getService(throughputStoreHandle);
 			const warmService = host.getService(cacheWarmHandle);
+			const compactionService = host.getService(compactionHandle);
 			const lspService = host.getService(lspServiceHandle);
 			const logger = host.logger;
 
@@ -70,6 +74,7 @@ export function createTransportHttpExtension(): Extension & {
 				credentialStore,
 				throughputStore,
 				warmService,
+				compactionService,
 				lspService,
 				logger,
 				emit: host.emit.bind(host),
diff --git a/packages/transport-http/src/seam.ts b/packages/transport-http/src/seam.ts
index 1c89a34..3c507dc 100644
--- a/packages/transport-http/src/seam.ts
+++ b/packages/transport-http/src/seam.ts
@@ -4,9 +4,14 @@ export type { CredentialStore } from "@dispatch/credential-store";
 export { credentialStoreHandle } from "@dispatch/credential-store";
 export type { LspServerStatus, LspService } from "@dispatch/lsp";
 export { lspServiceHandle } from "@dispatch/lsp";
-export type { SessionOrchestrator, WarmService } from "@dispatch/session-orchestrator";
+export type {
+	CompactionService,
+	SessionOrchestrator,
+	WarmService,
+} from "@dispatch/session-orchestrator";
 export {
 	cacheWarmHandle,
+	compactionHandle,
 	conversationOpened,
 	sessionOrchestratorHandle,
 } from "@dispatch/session-orchestrator";
diff --git a/packages/transport-http/src/server.bun.test.ts b/packages/transport-http/src/server.bun.test.ts
index a15a2c7..e770867 100644
--- a/packages/transport-http/src/server.bun.test.ts
+++ b/packages/transport-http/src/server.bun.test.ts
@@ -65,6 +65,11 @@ function fakeConversationStore(): ConversationStore {
 			return null;
 		},
 		async setConversationStatus() {},
+		async replaceHistory() {},
+		async getCompactThreshold() {
+			return null;
+		},
+		async setCompactThreshold() {},
 	};
 }