feat(cli): remove skill registry

[codeaholicguy]

Summary

• add ai-devkit skill remove-registry <id> with project-only removal by default
• make --global remove the global config entry and recursively delete that registry's cached repository
• validate registry IDs and require the resolved deletion target to remain inside ~/.ai-devkit/skills/
• keep the seed-backed discovery index unchanged because it intentionally catalogs unconfigured registries
• protect defaults structurally through selected-config own-property checks, with an explicit hard rejection for the built-in registry
• document the command and reviewed simplification in the user guide, changelog, and lifecycle records

Design and behavior

The command reads only the selected config map and delegates persistence planning to its config manager. Project removal keeps the cache. Global removal resolves both the cache root and registry target, verifies strict containment, removes the global registration, and deletes only that cache directory. The earlier focused index cleanup and frozen default-registry snapshot were removed as inconsistent and redundant.

Validation

• npm run build — passed for all 6 projects
• npm test — 1,954 tests passed across 140 files
• npm run lint — passed for all 6 projects with 0 errors (4 existing unused-catch warnings)
• targeted changed-module suites — 176 tests passed across 5 files
• planner module coverage — 100% statements, branches, functions, and lines
• feature lifecycle lint — passed
• built CLI help — verified --global documents cache deletion
• git diff --check — passed

Risks and follow-ups

• --global intentionally deletes ~/.ai-devkit/skills/<registry-id> after containment validation
• installed-skill locations are not traversed or removed
• registry command-group migration remains deferred