fix(#2736): prevent infinite compaction loop by setting cooldown before try

lastCompactionTime was only set on successful compaction. When compaction
failed (rate limit, timeout, etc.), no cooldown was recorded, causing
immediate retries in an infinite loop.

Fix: set lastCompactionTime before the try block so both success and
failure respect the cooldown window.

- test: add failed-compaction cooldown enforcement test
- test: fix timeout retry test to advance past cooldown

src/hooks/preemptive-compaction.ts

@@ -112,6 +112,7 @@ export function createPreemptiveCompactionHook(
     if (usageRatio < PREEMPTIVE_COMPACTION_THRESHOLD || !cached.modelID) return
 
     compactionInProgress.add(sessionID)
+    lastCompactionTime.set(sessionID, Date.now())
 
     try {
       const { providerID: targetProviderID, modelID: targetModelID } = resolveCompactionModel(
@@ -132,7 +133,6 @@ export function createPreemptiveCompactionHook(
       )
 
       compactedSessions.add(sessionID)
-      lastCompactionTime.set(sessionID, Date.now())
     } catch (error) {
       log("[preemptive-compaction] Compaction failed", { sessionID, error: String(error) })
     } finally {