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, 19 insertions, 0 deletions

diff --git a/packages/session-orchestrator/src/orchestrator.test.ts b/packages/session-orchestrator/src/orchestrator.test.ts
index e657fe2..d5585c2 100644
--- a/packages/session-orchestrator/src/orchestrator.test.ts
+++ b/packages/session-orchestrator/src/orchestrator.test.ts
@@ -95,6 +95,8 @@ function createInMemoryStore(): ConversationStore & {
 			return null;
 		},
 		async setCompactThreshold() {},
+		async forkHistory() {},
+		async setCompactedFrom() {},
 	};
 }
 
@@ -550,6 +552,8 @@ describe("turn-sealed event", () => {
 				return null;
 			},
 			async setCompactThreshold() {},
+			async forkHistory() {},
+			async setCompactedFrom() {},
 		};
 
 		const { orchestrator } = createSessionOrchestrator({
@@ -621,6 +625,8 @@ describe("turn-sealed event", () => {
 				return null;
 			},
 			async setCompactThreshold() {},
+			async forkHistory() {},
+			async setCompactedFrom() {},
 		};
 
 		const { orchestrator } = createSessionOrchestrator({
@@ -981,6 +987,8 @@ describe("turn metrics persistence", () => {
 				return null;
 			},
 			async setCompactThreshold() {},
+			async forkHistory() {},
+			async setCompactedFrom() {},
 		};
 
 		const { orchestrator } = createSessionOrchestrator({
diff --git a/packages/session-orchestrator/src/orchestrator.ts b/packages/session-orchestrator/src/orchestrator.ts
index c3b94bf..21c068c 100644
--- a/packages/session-orchestrator/src/orchestrator.ts
+++ b/packages/session-orchestrator/src/orchestrator.ts
@@ -132,6 +132,7 @@ export const conversationStatusChanged: EventHookDescriptor<ConversationStatusCh
 /** Payload for the conversationCompacted bus event. */
 export interface ConversationCompactedPayload {
 	readonly conversationId: string;
+	readonly archiveId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }
@@ -801,6 +802,11 @@ export function createCompactionService(
 				return { error: "model produced empty summary" };
 			}
 
+			// Non-destructive: fork the full pre-compaction history to an
+			// archive conversation before replacing it.
+			const archiveId = crypto.randomUUID();
+			await deps.conversationStore.forkHistory(conversationId, archiveId);
+
 			// Replace history: [system: summary] + recent messages
 			const summaryMessage: ChatMessage = {
 				role: "system",
@@ -813,15 +819,18 @@ export function createCompactionService(
 			};
 
 			await deps.conversationStore.replaceHistory(conversationId, [summaryMessage, ...toKeep]);
+			await deps.conversationStore.setCompactedFrom(conversationId, archiveId);
 
 			const result: CompactionResult = {
 				summary,
+				archiveId,
 				messagesSummarized: toSummarize.length,
 				messagesKept: toKeep.length,
 			};
 
 			deps.emit(conversationCompacted, {
 				conversationId,
+				archiveId,
 				messagesSummarized: toSummarize.length,
 				messagesKept: toKeep.length,
 			});
diff --git a/packages/session-orchestrator/src/queue.test.ts b/packages/session-orchestrator/src/queue.test.ts
index ae24ddf..346f9c4 100644
--- a/packages/session-orchestrator/src/queue.test.ts
+++ b/packages/session-orchestrator/src/queue.test.ts
@@ -91,6 +91,8 @@ function createInMemoryStore(): ConversationStore & {
 			return null;
 		},
 		async setCompactThreshold() {},
+		async forkHistory() {},
+		async setCompactedFrom() {},
 	};
 }