feat(skills): skills dependency resolution

[SurviveM]

Requirement

Skills declare requires: [skill-x, skill-y]. On import, auto-check and prompt for missing dependencies. Support one-click install of full dependency tree. Conflict detection for same-name different-version skills.

Verification

1. Import skill-A (requires skill-B) -> see prompt "skill-B missing, install now?" -> click confirm -> skill-B auto-installs -> skill-A status becomes "ready"
2. Uninstall skill-B -> skill-A status becomes "broken"
3. Import skill-C with same name but different version as existing -> conflict warning displayed