perf: cache comment router label lookups

[stainlu]

Caches comment-router open-label issue lookups per process so autofix/automerge durable comment discovery and command synthesis do not repeat identical GitHub searches. Validation: build:repair, comment-router-core test, targeted oxlint, targeted oxfmt, git diff --check.

[steipete]

The label-lookup cache is a small, sensible reduction in repeated GitHub calls, but the branch is dirty against current main. Please rebase and rerun pnpm run check before this can land.

The important behavior to preserve is per-process caching with copied return arrays, so callers cannot mutate cached state between lookups.

[stainlu]

Rebased on current main and reran pnpm run check under Node 24. The cache still returns copied arrays so callers cannot mutate cached label-number state.

[steipete]

Landed this manually after resolving the shared CHANGELOG.md conflict from the capacity-probe PR. The implementation and regression test stayed intact.

Landed on main: ec1ea902a3

Thanks @stainlu.