feat: add TTL-based file-backed caching for GitHubCLIProvider

[Achiever199]

Summary

Closes #35

Adds an optional caching layer to GitHubCLIProvider to avoid
redundant gh CLI calls within and across sessions.

Changes

New file: src/oss_dev/providers/cache.py

• CacheEntry — stores value + expiry timestamp
• ResponseCache — two-layer cache (memory + disk)
  • Memory layer avoids repeated disk reads in the same process
  • Disk layer (JSON files) survives process restarts
  • TTL-based expiry on all entries
  • make_key() uses SHA-256 hashing for safe filenames

Modified: src/oss_dev/providers/github/client.py

• Added _run_gh_cached() helper for read-only calls
• Cached methods: fetch_issue, list_issues,
  get_pr_status, get_pr_comments
• create_pr intentionally bypasses cache (mutating operation)
• Cache controlled via config.cache.enabled / ttl / dir

New file: tests/providers/test_cache.py

• 22 unit tests, all passing
• Covers: TTL expiry, disk persistence, disabled mode,
  invalidation, provider integration