feat: non-destructive compaction — fork history to archive before replacing

Compaction now preserves the full pre-compaction history:
1. Forks the conversation to a new archive ID (complete copy: chunks,
   metadata, cwd, reasoning-effort). Archive gets status=closed,
   title='Archive: <original>', compactedFrom=<originalId>.
2. Replaces the original conversation's history with [system: summary]
   + recent N messages (same as before).
3. Sets compactedFrom=<archiveId> on the original conversation's metadata.

The original history is never destroyed. The archive is accessible via
GET /conversations/:id using the archive ID.

Wire/contract changes:
- ConversationMeta: add compactedFrom?: string
- CompactionResult: add archiveId: string
- ConversationCompactedMessage: add archiveId
- CompactResponse: add archiveId

Conversation store:
- forkHistory(sourceId, targetId): copies all chunks + metadata to a
  new conversation ID
- setCompactedFrom(conversationId, archiveId): marks the conversation

3 files changed, 13 insertions, 0 deletions

diff --git a/packages/transport-http/src/app.test.ts b/packages/transport-http/src/app.test.ts
index 51f791f..b265f5e 100644
--- a/packages/transport-http/src/app.test.ts
+++ b/packages/transport-http/src/app.test.ts
@@ -143,6 +143,8 @@ function createFakeConversationStore(
 			return null;
 		},
 		async setCompactThreshold() {},
+		async forkHistory() {},
+		async setCompactedFrom() {},
 	};
 }
 
@@ -869,6 +871,8 @@ describe("GET /conversations/:id", () => {
 					return null;
 				},
 				async setCompactThreshold() {},
+				async forkHistory() {},
+				async setCompactedFrom() {},
 			};
 			const app = createApp({
 				conversationStore: store,
@@ -942,6 +946,8 @@ describe("GET /conversations/:id", () => {
 				return null;
 			},
 			async setCompactThreshold() {},
+			async forkHistory() {},
+			async setCompactedFrom() {},
 		};
 		const app = createApp({
 			conversationStore: store,
@@ -1084,6 +1090,8 @@ describe("GET /conversations/:id/metrics", () => {
 				return null;
 			},
 			async setCompactThreshold() {},
+			async forkHistory() {},
+			async setCompactedFrom() {},
 		};
 		const app = createApp({
 			conversationStore: brokenStore,
@@ -2059,6 +2067,8 @@ describe("PUT /conversations/:id/reasoning-effort", () => {
 				return null;
 			},
 			async setCompactThreshold() {},
+			async forkHistory() {},
+			async setCompactedFrom() {},
 		};
 		const app = createApp({
 			conversationStore: store,
diff --git a/packages/transport-http/src/app.ts b/packages/transport-http/src/app.ts
index fd78f3e..233e30e 100644
--- a/packages/transport-http/src/app.ts
+++ b/packages/transport-http/src/app.ts
@@ -725,6 +725,7 @@ export function createApp(opts: CreateServerOptions): Hono {
 
 		const response: CompactResponse = {
 			conversationId,
+			archiveId: result.archiveId,
 			messagesSummarized: result.messagesSummarized,
 			messagesKept: result.messagesKept,
 		};
diff --git a/packages/transport-http/src/server.bun.test.ts b/packages/transport-http/src/server.bun.test.ts
index e770867..3b7700e 100644
--- a/packages/transport-http/src/server.bun.test.ts
+++ b/packages/transport-http/src/server.bun.test.ts
@@ -70,6 +70,8 @@ function fakeConversationStore(): ConversationStore {
 			return null;
 		},
 		async setCompactThreshold() {},
+		async forkHistory() {},
+		async setCompactedFrom() {},
 	};
 }