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.

scripts/build.ts

@@ -33,7 +33,7 @@ const featureFlags: Record<string, boolean> = {
   BUILTIN_EXPLORE_PLAN_AGENTS: true,
   CONTEXT_COLLAPSE: false,
   COMMIT_ATTRIBUTION: false,
-  TEAMMEM: false,
+  TEAMMEM: true,
   UDS_INBOX: false,
   BG_SESSIONS: false,
   AWAY_SUMMARY: false,