Add managed Skills lifecycle

[0xLeif]

Summary

• add safe list/status JSON plus managed update and uninstall commands
• complete agent metadata and rewrite the README as a release-ready catalog
• dogfood all 15 skills on Codex, Claude, and Cursor placements
• harden public-only guidance after verifying Let 0.2 project scope can expose federated local metadata

Test Plan

• fledge lanes validate . --strict
• fledge lanes run verify
• fledge lanes run audit
• skill-creator quick validation for 15 of 15 skills
• fresh Fledge plugin dogfood: 15 current installs, 15 update previews, and safe uninstall on each supported host
• CI YAML and embedded dogfood shell syntax validation

Known boundary

Let 0.2 project-scoped discovery can still enumerate user-global or sibling-worktree metadata. The catalog now blocks Let in public-only workflows until repository isolation is independently verified and uses public repository evidence as the safe fallback.

[0xLeif]

Superseded by #11, which merged the lifecycle work with the reviewed provider integration.