refactor(usage): address round-2 review on PR #871

- remove accidentally-committed .claude/scheduled_tasks.lock (Claude Code
  runtime artifact swept in by `git add -A`) and gitignore it; the rest
  of .claude/ stays tracked.
- store: cache UsageEventStore.shared() as a process-wide instance so the
  hot search path doesn't allocate a fresh wrapper per metered query (the
  wrapper is stateless beyond its storage handle).
- hooks: pass enabled=True directly (the outer guard already confirmed
  the flag) instead of re-reading settings.usage_metering_enabled.
- migration: document the no-TTL retention design (control-plane rollup
  owns the lifecycle; the data plane only appends).
- tests: assert the best-effort error path logs at WARNING (observability
  contract).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -10,6 +10,9 @@ __pycache__/
 settings.toml
 settings.local.toml
 
+# Claude Code runtime artifacts (the rest of .claude/ is tracked)
+.claude/scheduled_tasks.lock
+
 # Git
 worktrees/