fix: reduce session.messages() calls with event-based caching to prevent memory leaks

- Replace session.messages() fetch in context-window-monitor with message.updated event cache
- Replace session.messages() fetch in preemptive-compaction with message.updated event cache
- Add per-session transcript cache (5min TTL) to avoid full rebuild per tool call
- Remove session.messages() from background-agent polling (use event-based progress)
- Add TTL pruning to todo-continuation-enforcer session state Map
- Add setInterval.unref() to tool-input-cache cleanup timer

Fixes #1222

src/hooks/claude-code-hooks/tool-input-cache.ts

@@ -37,7 +37,7 @@ export function getToolInput(
 }
 
 // Periodic cleanup (every minute)
-setInterval(() => {
+const cleanupInterval = setInterval(() => {
   const now = Date.now()
   for (const [key, entry] of cache.entries()) {
     if (now - entry.timestamp > CACHE_TTL) {
@@ -45,3 +45,7 @@ setInterval(() => {
     }
   }
 }, CACHE_TTL)
+// Allow process to exit naturally even if interval is running
+if (typeof cleanupInterval === "object" && "unref" in cleanupInterval) {
+  cleanupInterval.unref()
+}