feat: Support new owner/repo/skill install format with REST-first

[RichardHightower]

Summary

This PR implements support for the NEW install format owner/repo/skill as the primary format while maintaining backwards compatibility with the LEGACY format owner_repo/skill.

Changes Made

🔧 Core API Client Updates (src/skilz/api_client.py)

• Replaced parse_skill_id() to support THREE formats:
  • NEW: owner/repo/skill (2 slashes) - Primary format
  • LEGACY: owner_repo/skill (1 slash with underscore) - Backwards compatible
  • SLUG: owner__repo__skill (double underscores) - Direct Firestore doc ID
• Updated is_marketplace_skill_id() to recognize all three formats
• Added get_skill_id_format() to detect format type

📊 Enhanced Logging (src/skilz/installer.py)

• Added verbose logging to show skill ID format detection
• Added logging for REST API attempts and success
• Shows whether REST API or GitHub fallback was used

✅ Comprehensive Tests (tests/test_api_client.py)

• Added TestParseSkillIdNewFormat - Tests NEW format parsing
• Added TestParseSkillIdSlugFormat - Tests SLUG format parsing
• Added TestIsMarketplaceSkillIdFormats - Tests format recognition
• Added TestGetSkillIdFormat - Tests format detection
• Fixed existing test for updated error messages

Format Support Matrix

Format	Example	Resolution Method
NEW	skilz install owner/repo/skill	REST API FIRST → GitHub fallback
LEGACY	skilz install owner_repo/skill	REST API first → GitHub fallback
SLUG	skilz install owner__repo__skill	REST API first → GitHub fallback
Git URL	skilz install -g https://...	Direct GitHub clone (no REST)

Verbose Output Examples

NEW Format

$ skilz -v install davila7/claude-code-templates/slack-gif-creator --agent claude
Using specified agent: Claude Code
Looking up skill: davila7/claude-code-templates/slack-gif-creator
  [INFO] Skill ID format: NEW
  [INFO] Attempting REST API lookup at skillzwave.ai...
  [SUCCESS] REST API resolved skill: davila7/claude-code-templates/slack-gif-creator

LEGACY Format

$ skilz -v install davila7_claude-code-templates/slack-gif-creator --agent claude
  [INFO] Skill ID format: LEGACY
  [INFO] Attempting REST API lookup at skillzwave.ai...
  [SUCCESS] REST API resolved skill: davila7_claude-code-templates/slack-gif-creator

Invalid Format

$ skilz -v install invalid-format --agent claude
  [INFO] Skill ID format: UNKNOWN
  # No REST API attempt for unknown formats

Testing Results

✅ All 633 tests passing
✅ 25 new API client tests added
✅ Linting and type checking clean
✅ Manual testing verified for all formats

Backwards Compatibility

• ✅ LEGACY format (owner_repo/skill) continues to work unchanged
• ✅ Existing installations remain functional
• ✅ All existing tests continue to pass

Resolution Flow

User: skilz install owner/repo/skill --verbose

1. Detect format → "NEW" 
2. Log: "[INFO] Skill ID format: NEW"
3. Log: "[INFO] Attempting REST API lookup at skillzwave.ai..."
4. Call: GET /api/skills/byname?repoFullName=owner/repo&name=skill
   ├─ 200 OK → Log: "[SUCCESS] REST API resolved skill" → Install
   ├─ 404 Not Found → Log: "[WARN] REST API failed, falling back to GitHub..."
   └─ 400 Bad Request → Log: "[ERROR] Invalid skill identifier format" → Exit

Success Criteria Met

• src/skilz/api_client.py has been MODIFIED with new function implementations
• src/skilz/installer.py has logging for REST vs GitHub resolution
• tests/test_api_client.py has new test classes added
• All tests pass (633/633)
• NEW format resolves correctly via REST API
• LEGACY format resolves correctly via REST API
• SLUG format resolves correctly via REST API
• Verbose output shows whether REST or GitHub was used
• Quality checks pass (linting, type checking)

Closes SKILZ-FORMAT-001