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

1 files changed, 9 insertions, 0 deletions

diff --git a/packages/wire/src/index.ts b/packages/wire/src/index.ts
index 69f35f0..8c85f89 100644
--- a/packages/wire/src/index.ts
+++ b/packages/wire/src/index.ts
@@ -521,6 +521,12 @@ export interface ConversationMeta {
 	readonly lastActivityAt: number;
 	readonly title: string;
 	readonly status: ConversationStatus;
+	/**
+	 * Set on a compacted conversation: points to the archive conversation ID
+	 * that holds the full pre-compaction history. Absent on conversations
+	 * that have never been compacted.
+	 */
+	readonly compactedFrom?: string;
 }
 
 // ─── Compaction ──────────────────────────────────────────────────────────────
@@ -529,9 +535,12 @@ export interface ConversationMeta {
  * Result of a compaction operation. `summary` is the text the model produced;
  * `messagesKept` is how many recent messages were retained after the summary;
  * `messagesSummarized` is how many old messages were replaced by the summary.
+ * `archiveId` is the ID of the new conversation that holds the full
+ * pre-compaction history (non-destructive — the original history is preserved).
  */
 export interface CompactionResult {
 	readonly summary: string;
+	readonly archiveId: string;
 	readonly messagesSummarized: number;
 	readonly messagesKept: number;
 }