feat(story-3.4): implement server concurrency throttling and freshness check
Add --check-freshness flag (index freshness gate blocking stale data),
--batch flag (sequential for...of query processing, anti-Promise.all),
and freshness metadata in list_repos output.
Code review patches applied:
- Reset 7 patch findings (AC #4 real diagnostic, null guards,
shutdown separation, empty target validation)
- Resolved 3 deferred edge cases (Date.parse NaN, falsy 0, type guard)
- Resolved 4 pre-existing issues (infinite while-loop guard,
resolveRepoId cross-platform, empty fallback, JSON error logging)
Closes story-3.4
72e293f346