Update docs for recent feature additions

[devrimcavusoglu]

Summary

• Add skill edit command section to docs/reference/commands.md
• Document new flags: --tags on create, --tag filter on list/search, --force on install
• Add tags field to skill format spec (frontmatter table + example YAML)
• Document stylistic lint hints and parse warnings in docs/reference/validation.md
• Fix Go version requirement from 1.23+ to 1.25+ in docs/guide/installation.md
• Remove completed skill update item from AGENTS.md future roadmap

Test plan

• Verify all 5 changed files render correctly
• Cross-check documented flags against CLI --help output
• Confirm no broken internal doc links

🤖 Generated with Claude Code

[devrimcavusoglu]

Verification

• CI: all checks pass (lint, test, docs-build on both ubuntu/macos)
• Cross-checked all documented flags against --help output for skill create, skill edit, skill search, skill list, skill install — all match
• Docs-only PR, no new test cases needed (features tested in PRs Add --force flag to skill install (#33) #60–Add skill edit/update command (#39) #64)
• Fixed one pre-existing gap: added missing --scope flag to skill create docs (a26a9f8)

LGTM