fix: compaction keeps original ID, forks old history to archive, chains via compactedFrom

Reworked compaction to match the confirmed design:
- The compacted conversation KEEPS its original ID (messaging between
  agents is unaffected — the ID never changes)
- The old full history is forked to a new archive conversation (new UUID)
- The archive inherits the source's compactedFrom, creating a chain:
  A → Y → X (walk compactedFrom backward)
- A's history is replaced with [summary + recent N]
- A.compactedFrom = archive ID

forkHistory: inherit compactedFrom from source (not set to sourceId),
so archives chain backward to previous archives.

FE: no tab switching needed — the ID doesn't change. Just reload history.

1 files changed, 8 insertions, 5 deletions

diff --git a/packages/session-orchestrator/src/orchestrator.ts b/packages/session-orchestrator/src/orchestrator.ts
index 21c068c..b46ecc1 100644
--- a/packages/session-orchestrator/src/orchestrator.ts
+++ b/packages/session-orchestrator/src/orchestrator.ts
@@ -132,7 +132,7 @@ export const conversationStatusChanged: EventHookDescriptor<ConversationStatusCh
 /** Payload for the conversationCompacted bus event. */
 export interface ConversationCompactedPayload {
 	readonly conversationId: string;
-	readonly archiveId: string;
+	readonly newConversationId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }
@@ -802,8 +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.
+			// Non-destructive: fork the full pre-compaction history to a new
+			// archive conversation. The original conversation keeps its ID
+			// (so messaging between agents still works) and gets the compacted
+			// content. The archive inherits the original's compactedFrom,
+			// creating a chain: A → Y → X → ...
 			const archiveId = crypto.randomUUID();
 			await deps.conversationStore.forkHistory(conversationId, archiveId);
 
@@ -823,14 +826,14 @@ export function createCompactionService(
 
 			const result: CompactionResult = {
 				summary,
-				archiveId,
+				newConversationId: archiveId,
 				messagesSummarized: toSummarize.length,
 				messagesKept: toKeep.length,
 			};
 
 			deps.emit(conversationCompacted, {
 				conversationId,
-				archiveId,
+				newConversationId: archiveId,
 				messagesSummarized: toSummarize.length,
 				messagesKept: toKeep.length,
 			});