fix(skills): add 429 rate limit retry to GitHub Contents API fallback

[0xbyt4]

Summary

When the Git Trees API falls back to the recursive Contents API (truncated tree or API unavailable), HTTP 429 rate-limited responses caused silent subdirectory loss:

• Files were skipped with only a DEBUG log
• The skill was recorded as fully installed
• Users ended up with broken/incomplete skills without any warning

This adds retry with Retry-After header support to both _download_directory_recursive and _fetch_file_content:

• 3 attempts with configurable backoff (from Retry-After header, capped at 15s)
• Guards against non-numeric Retry-After values (HTTP-date format)
• Matches the existing retry pattern in ClawHub._download_zip (line ~1729)

Log levels follow codebase convention:

• debug: individual retry attempts, non-200 responses, network errors
• info: tree truncation (normal for large repos)
• warning: only when all 3 retries exhausted (actual data loss risk)

Test plan

• tests/tools/test_skills_hub.py — 73/73 passed
• Verified retry loop for/else pattern is correct
• Verified int(Retry-After) guarded with try/except for non-numeric headers
• Verified empty subdirectories don't trigger false "failed" warnings

[teknium1]

Thanks for identifying the silent Contents API fallback failure mode. This is an automated hermes-sweeper review; the implementation has already landed on main via a salvage that preserves the intent and contributor credit.

• a0c90edf48a1f8e6bf8a08823fedc680e5d89663 (fix(skills): retry rate-limited Contents API directory listings) explicitly salvages PR fix(skills): add 429 rate limit retry to GitHub Contents API fallback #3033's intent and is merged through d51657c0c6ae725ca05406800f92561b9965859a / PR fix(skills): retry rate-limited Contents API directory listings (salvage #3033) #58531.
• tools/skills_hub.py:914 routes recursive Contents API directory listings through _github_get.
• tools/skills_hub.py:973 routes individual Contents API file downloads through the same helper.
• tools/skills_hub.py:769-840 provides three-attempt 403/429 retry handling, including numeric Retry-After and reset-header backoff.

This shipped in v2026.7.7.