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

3 files changed, 64 insertions, 0 deletions

diff --git a/packages/cli/src/args.ts b/packages/cli/src/args.ts
index 30fa309..ecf6e2e 100644
--- a/packages/cli/src/args.ts
+++ b/packages/cli/src/args.ts
@@ -34,6 +34,7 @@ export type ParsedCommand =
 			readonly status?: string;
 			readonly all: boolean;
 	  }
+	| { readonly kind: "compact"; readonly server: string; readonly conversationId: string }
 	| { readonly kind: "open"; readonly server: string; readonly conversationId: string }
 	| { readonly kind: "read"; readonly server: string; readonly conversationId: string }
 	| {
@@ -108,6 +109,28 @@ export function parseArgs(argv: readonly string[], opts: ParseOpts): ParsedComma
 		};
 	}
 
+	if (first === "compact") {
+		let server = opts.defaultServer;
+		let conversationId: string | undefined;
+		for (let i = 1; i < argv.length; i++) {
+			const arg = argv[i] as string;
+			if (arg === "--server") {
+				if (i + 1 >= argv.length) return { kind: "error", message: "--server requires a value" };
+				server = argv[++i] as string;
+			} else if (arg.startsWith("--")) {
+				return { kind: "error", message: `Unknown flag: ${arg}` };
+			} else if (conversationId !== undefined) {
+				return { kind: "error", message: `Unexpected argument for 'compact': ${arg}` };
+			} else {
+				conversationId = arg;
+			}
+		}
+		if (conversationId === undefined) {
+			return { kind: "error", message: "'compact' requires a conversation id" };
+		}
+		return { kind: "compact", server, conversationId };
+	}
+
 	if (first === "read") {
 		let server = opts.defaultServer;
 		let conversationId: string | undefined;
diff --git a/packages/cli/src/http.ts b/packages/cli/src/http.ts
index 876f570..585c678 100644
--- a/packages/cli/src/http.ts
+++ b/packages/cli/src/http.ts
@@ -10,6 +10,7 @@
 import type {
 	AgentEvent,
 	ChatRequest,
+	CompactResponse,
 	ConversationListResponse,
 	LastMessageResponse,
 	ModelsResponse,
@@ -183,6 +184,26 @@ export async function openConversation(
 	return (await res.json()) as OpenConversationResponse;
 }
 
+interface CompactConversationOpts {
+	readonly server: string;
+	readonly conversationId: string;
+}
+
+export async function compactConversation(
+	deps: FetchDeps,
+	opts: CompactConversationOpts,
+): Promise<CompactResponse> {
+	const url = `${opts.server}/conversations/${encodeURIComponent(opts.conversationId)}/compact`;
+	const res = await deps.fetchImpl(url, { method: "POST" });
+
+	if (!res.ok) {
+		const body = await res.text();
+		throw new Error(`POST /conversations/:id/compact failed with status ${res.status}: ${body}`);
+	}
+
+	return (await res.json()) as CompactResponse;
+}
+
 /**
  * The outcome of short-ID resolution: either the full conversation id to use,
  * or a human-readable error describing why resolution failed.
diff --git a/packages/cli/src/main.ts b/packages/cli/src/main.ts
index e709f21..4e07da9 100644
--- a/packages/cli/src/main.ts
+++ b/packages/cli/src/main.ts
@@ -9,6 +9,7 @@ import { readFile } from "node:fs/promises";
 import { parseArgs } from "./args.js";
 import { formatCatalog } from "./catalog.js";
 import {
+	compactConversation,
 	enqueueMessage,
 	fetchConversations,
 	fetchLastMessage,
@@ -23,6 +24,7 @@ import { extractLastText, formatConversationList, renderEvent } from "./render.j
 const USAGE = `Usage:
   dispatch models [--server <url>]
   dispatch list [<prefix>] [--status <active|idle|closed>] [--all] [--server <url>]
+  dispatch compact <conversationId> [--server <url>]
   dispatch read <conversationId> [--server <url>]
   dispatch open <conversationId> [--server <url>]
   dispatch send <conversationId> --text "..." [--queue] [--open] [--cwd <dir>] [--effort <level>] [--server <url>]
@@ -79,6 +81,24 @@ async function main(): Promise<void> {
 			if (last.content.length > 0) process.stdout.write(`${last.content}\n`);
 			break;
 		}
+		case "compact": {
+			const resolved = await resolveConversationId(
+				{ fetchImpl: globalThis.fetch },
+				{ server: parsed.server, shortId: parsed.conversationId },
+			);
+			if (typeof resolved !== "string") {
+				process.stderr.write(`${resolved.error}\n`);
+				process.exit(1);
+			}
+			const result = await compactConversation(
+				{ fetchImpl: globalThis.fetch },
+				{ server: parsed.server, conversationId: resolved },
+			);
+			process.stdout.write(
+				`Compacted ${resolved}: ${result.messagesSummarized} messages summarized, ${result.messagesKept} kept.\n`,
+			);
+			break;
+		}
 		case "open": {
 			const resolved = await resolveConversationId(
 				{ fetchImpl: globalThis.fetch },