Add skills feature with code quality improvements

[HartBrook]

Summary

• Add skills management feature implementing the Agent Skills standard (agentskills.io) with Claude Code extensions
• Include starter skills: code-review, security-audit, test-gen
• Improve code quality across the codebase based on code review findings

Changes

Skills Feature

• New internal/skills package for skill parsing, registry, and syncing
• CLI command stag skills for listing and managing skills
• Skills sync from team repos during stag sync
• Support for skill installation to ~/.claude/skills/ or .claudeskills/
• Hierarchical precedence: project > personal > team > starter

Code Quality Improvements

• Error handling: Fixed swallowed error in YAML marshal (claude.go)
• Context propagation: GitHub client methods now properly use context for cancellation/timeouts
• Code deduplication: Extracted syncDirectoryContents helper to reduce repeated sync logic
• Consistent logging: Changed raw stderr writes to log.Printf for library code
• Better errors: Added directory paths to error messages for easier debugging
• Configurable threshold: Token warning threshold now uses config instead of hardcoded value

Test plan

• All existing tests pass (go test ./...)
• Skills parsing handles valid and invalid frontmatter
• Registry correctly handles precedence across sources
• Integration tests verify end-to-end sync behavior
• Manual testing of stag skills command
• Manual testing of skills sync during stag sync