feat(cli): harden per-registry skill updates

[codeaholicguy]

Summary

The optional per-registry skill update capability and exact owner/repo filtering already existed on main. This hardening follow-up adds direct SkillRegistry contract coverage and improves the selector UX without reimplementing the baseline.

• add dedicated temporary-cache tests for all-registry and exact filtering, non-Git skips, summary counts, and failure handling
• list sorted available registry IDs in NotFoundError messages
• resolve a unique owner-less repository shorthand and report the selected full ID; reject zero or ambiguous matches
• document per-registry usage and add lifecycle/changelog records

Validation

• npm ci
• npm run build — 6/6 projects
• npm run lint — 6/6 projects, warnings only
• npm test — 1,923 tests across 137 files
• focused SkillRegistry.test.ts — 9/9 tests
• focused coverage — 64.42% statements / 62% branches for the whole SkillRegistry.ts, including unrelated fetch/clone paths; all added selector branches exercised
• npx ai-devkit@latest lint --feature skill-update-registry
• CLI help smoke test for optional [registry-id]

Risks

None known. No public API, schema, cache layout, or command parser changes.