feat: activate local-only team memory in open build (#648)

* feat: activate local-only team memory in open build

Enable the TEAMMEM feature flag and the isTeamMemoryEnabled() gate so
team memory works in local-only mode for all open-build users.

Team memory is a shared memory system scoped per-project, stored at
~/.claude/projects/<project>/memory/team/. The implementation is
already almost entirely local — extraction, UI, prompts, file
detection, and path validation all work on local files.

The cloud sync overlay (OAuth + API) is cleanly separated: the
watcher does an early return when OAuth is unavailable, so the
feature degrades gracefully to local-only storage with no crashes.

What works locally:
- Memory extraction (auto + team, combined prompts)
- Team MEMORY.md loaded into conversation context
- File selector with team memory folder option
- Collapse tracking (read/search/write counts)
- Secret scanning before persistence
- Path validation + symlink protection

What requires OAuth (not available in open build):
- Cloud sync between team members
- Automatic push/pull via file watcher

* fix: preserve opt-out gate for team memory via feature flag

Change isTeamMemoryEnabled() to read tengu_herring_clock with default
true instead of unconditional return true. This enables team memory by
default while preserving user opt-out via ~/.claude/feature-flags.json.

src/memdir/teamMemPaths.ts

@@ -74,7 +74,7 @@ export function isTeamMemoryEnabled(): boolean {
   if (!isAutoMemoryEnabled()) {
     return false
   }
-  return getFeatureValue_CACHED_MAY_BE_STALE('tengu_herring_clock', false)
+  return getFeatureValue_CACHED_MAY_BE_STALE('tengu_herring_clock', true)
 }
 
 /**