fix(todo-continuation-enforcer): tighten post-compaction guard with session-agent fallback

Refine continuation agent resolution to prefer session-state agent fallback while keeping compaction-specific protection. Replace sticky boolean compaction flag with a short-lived timestamp guard so unresolved agents are blocked only during the immediate post-compaction window, avoiding long-lived suppression and preserving existing continuation behavior.

src/hooks/todo-continuation-enforcer/compaction-guard.ts

@@ -0,0 +1,10 @@
+import { COMPACTION_GUARD_MS } from "./constants"
+import type { SessionState } from "./types"
+
+export function isCompactionGuardActive(state: SessionState, now: number): boolean {
+  if (!state.recentCompactionAt) {
+    return false
+  }
+
+  return now - state.recentCompactionAt < COMPACTION_GUARD_MS
+}