feat(signals): advisory reviewer routing via CODEOWNERS + load

[JSONbored]

Parent roadmap: #525
Parent phase: #528

Background

Maintainers spend effort deciding who should review a PR. Gittensory can suggest reviewers from CODEOWNERS, de-weighted by current load — advisory by default.

Goal

Advisory reviewer routing surfaced in the PR panel / maintainer-packet.

Current Behavior

No CODEOWNERS parsing or reviewer-routing references exist in src.

Desired Behavior

• New src/github/codeowners.ts parser (fetch .github/CODEOWNERS via existing contents:read) + src/signals/reviewer-routing.ts ranking owners de-weighted by buildBurdenForecast load.
• Surface ranked suggestions in the panel + maintainer-packet. Auto-request reviewers only when reviewerRoutingMode=auto_request.

Implementation Requirements

• Cache CODEOWNERS per repo+ref. Load shown as a band (light/busy), not raw counts.

Public/Private Output Boundaries

• Reviewer logins are public repo metadata; no private scoring implied. Never auto-request for first-time external contributors without opt-in.

Acceptance Criteria

• Suggestions match changed-file ownership and reflect load; auto-request strictly opt-in.

Testing Requirements

• npm run test:ci, 97%+ coverage. CODEOWNERS parser tests + ranking tests + opt-in guard test.