Add skill tags/categories to frontmatter (#41)

[devrimcavusoglu]

Summary

• Add optional tags field to Skill struct and YAML frontmatter
• Add --tags flag to skill create for setting tags at creation time
• Add --tag filter to skill list and skill search for filtering by tag
• Display tags in skill show output and include in JSON output

Test plan

• make lint && make test passes
• TestManifest_Roundtrip_Tags, TestParseManifest_WithTags, TestParseManifest_NoTags verify manifest parsing
• TestSkillCreate_WithTags, TestSkillCreate_WithTags_Show verify creation with tags
• TestSkillList_FilterByTag, TestSkillSearch_FilterByTag verify filtering

Closes #41

🤖 Generated with Claude Code

[devrimcavusoglu]

Verified:

• make lint && make test — 0 lint issues, all packages pass
• Manifest: TestManifest_Roundtrip_Tags, TestParseManifest_WithTags, TestParseManifest_NoTags all pass
• Create: TestSkillCreate_WithTags (JSON) and TestSkillCreate_WithTags_Show (text) cover --tags flag and output
• Filter: TestSkillList_FilterByTag (3 skills, correct 2 returned) and TestSkillSearch_FilterByTag (2 skills, correct 1 returned) cover --tag on both commands
• All new features have CLI-level coverage, no gaps found