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, 41 insertions, 1 deletions

diff --git a/packages/transport-contract/src/index.ts b/packages/transport-contract/src/index.ts
index 237f4ab..86eb807 100644
--- a/packages/transport-contract/src/index.ts
+++ b/packages/transport-contract/src/index.ts
@@ -32,6 +32,7 @@ import type {
 
 export type {
 	AgentEvent,
+	CompactionResult,
 	ConversationMeta,
 	ConversationStatus,
 	QueuedMessage,
@@ -486,7 +487,8 @@ export type WsServerMessage =
 	| ChatDeltaMessage
 	| ChatErrorMessage
 	| ConversationOpenMessage
-	| ConversationStatusChangedMessage;
+	| ConversationStatusChangedMessage
+	| ConversationCompactedMessage;
 
 // ─── Conversation list + metadata ────────────────────────────────────────────
 
@@ -512,6 +514,18 @@ export interface ConversationStatusChangedMessage {
 }
 
 /**
+ * Broadcast to all connected WS clients when a conversation's history has been
+ * compacted (summarized). The frontend should reload the conversation history
+ * via `GET /conversations/:id` to reflect the compacted state.
+ */
+export interface ConversationCompactedMessage {
+	readonly type: "conversation.compacted";
+	readonly conversationId: string;
+	readonly messagesSummarized: number;
+	readonly messagesKept: number;
+}
+
+/**
  * Response for `GET /conversations` — the list of all known conversations,
  * sorted by `lastActivityAt` descending (most recent first). Each entry carries
  * enough metadata for a conversation picker UI (id, title, timestamps).
@@ -555,3 +569,29 @@ export interface TitleResponse {
 	readonly conversationId: string;
 	readonly title: string;
 }
+
+/**
+ * Response for `POST /conversations/:id/compact` — confirms the conversation
+ * history was compacted (old messages summarized, recent messages retained).
+ */
+export interface CompactResponse {
+	readonly conversationId: string;
+	readonly messagesSummarized: number;
+	readonly messagesKept: number;
+}
+
+/**
+ * Response for `GET /conversations/:id/compact-threshold` — the token count
+ * at which automatic compaction triggers (0 = manual only).
+ */
+export interface CompactThresholdResponse {
+	readonly conversationId: string;
+	readonly threshold: number;
+}
+
+/**
+ * Request body for `PUT /conversations/:id/compact-threshold`.
+ */
+export interface SetCompactThresholdRequest {
+	readonly threshold: number;
+}