Implement the github package client with Retry-After header parsing that
supports both integer seconds (e.g. "Retry-After: 120") and HTTP-date
format (e.g. "Retry-After: Thu, 01 Dec 2025 16:00:00 GMT") per RFC 7231
§7.1.3.
Key design decisions:
- Use http.ParseTime which handles RFC 1123, RFC 850, and ASCTIME formats
- Cap maximum retry delay at 60s (maxRetryAfter) to prevent stalling
- If HTTP-date is in the past, use delay of 0 (retry immediately)
- Inject time.Now via c.now field for deterministic testing
Closes#94
claw