[smart-log] feat: add rate limit retry logic

[luqven]

Summary

Add automatic retry with exponential backoff for GitHub API rate limits (HTTP 429).

Changes

Modified: src/api/mod.rs

• RateLimitError struct with reset time info and Display impl for user-friendly messages
• parse_rate_limit_headers() - Extract X-RateLimit-* headers from response
• is_rate_limited() - Detect rate limiting (429 or 403 with rate limit headers)
• send_with_retry() - Automatic retry with exponential backoff

Behavior

• Retries up to 3 times with 1s, 2s, 4s delays
• Handles both HTTP 429 and GitHub's 403-with-rate-limit-headers pattern
• Returns RateLimitError with helpful message: "Try again in X minutes"

Tests

6 new tests:

• test_rate_limit_error_display_with_reset / without_reset
• test_send_with_retry_success_first_try
• test_send_with_retry_rate_limit_then_success
• test_send_with_retry_exhausted
• test_send_with_retry_403_with_rate_limit

Stacked PR Chain: smart-log

PR	Title	Merges Into
#35	feat: add stack discovery API	-
#36	👉feat: add rate limit retry logic	#35
#37	feat: add identifier detection and prompts	#36
#38	feat: add gh CLI helpers	#37
#39	feat: make log and status identifier optional with branch inference	#38
#40	docs: update log and status documentation for smart defaults	#39
#41	perf: optimize stack discovery with batch fetch	#40
#42	perf: parallelize status check fetches	#41

[luqven]

Landed via #42