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

1 files changed, 21 insertions, 0 deletions

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.