feat: rate-limit detection, back off until reset

[masonlet]

Summary

Distinguishes GitHub rate-limiting from other fetch failures. A rate-limited 403 was previously handled identically to permission-denied or network errors; same generic log, same hardcoded 5-minute stale-cache window.

Changes

• Detect rate limiting (403/429 with X-RateLimit-Remaining: 0, or a Retry-After header) and log a distinct GitHub rate limit exceeded; resets at HH:MM:SS message once per refresh
• Stale-cache retry now waits until the actual reset time, clamped to [60s, refresh interval]; non-rate-limit failures keep the 5-minute fallback
• Split src/github.ts into src/github/ (fetch, parse, process, rateLimit, types) with tests mirrored
• Fix mockErrorResponse missing a headers stub, which silently bypassed the error path in existing tests
• Fix rateLimitReset returning 0 instead of null when the reset header is absent
• README notes the failure/backoff behavior