This commit implements User Story 5 from v0.2, enabling discovery of skills
in nested directory structures up to 5 levels deep with security safeguards.

Changes:
- Updated SkillDiscovery.find_skill_files() to use recursive directory walking
- Added _find_skill_files_recursive() helper with depth limit validation
- Implemented circular symlink detection using inode tracking
- Added depth limit warnings when exceeding 5 levels
- Updated async version afind_skill_files() to delegate to sync implementation
- Created example nested skill structure with 4 test skills at various depths
- Created test_nested_discovery.py validation script (all tests pass)

Features:
- Supports both flat and nested directory structures simultaneously
- Depth limit: warns when skills are deeper than 5 levels
- Security: detects and prevents circular symlinks
- Symlink resolution: resolves symlinks to canonical paths before validation
- Backward compatible: max_depth parameter defaults to 5

Test Results:
- Sync Discovery: ✅ PASSED (4/4 skills discovered)
- Async Discovery: ✅ PASSED (4/4 skills discovered)
- Sync/Async Consistency: ✅ PASSED (identical results)

Code Quality:
- All ruff checks passed
- All mypy strict type checks passed
- Full type annotations for new set[tuple[int, int]] parameter

Task Status:
- [X] T044: Recursive directory walking implemented
- [X] T045: Depth limit validation (warn if >5 levels)
- [X] T046: Skill name extraction (already correct from YAML frontmatter)
- [X] T047: Symlink resolution and circular detection
- [X] T048: Sync/async consistency validated
- [X] T049: Example nested structure created

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>