fix skim/vouch beacon ordering, add tests for skills feature
Fix skim to require beacon in default mode (matching original behavior),
only skip beacon check in --skills-only mode. Fix vouch to check beacon
before trusted for cap refs (matching original behavior).
Tests added:
- skill-ref: name validation, ref format, round-trip conversion
- ship --skill: SKILL.md parsing, name validation, no-beacon requirement
- learn: trust gate (vet required, skip-perms bypass, --user always requires vet)
- skim --skills: beacon requirements per mode
- vet skill: ref format, human-only gate, no-beacon for skills
- vouch skill: ref format, no-beacon for skills, beacon for caps
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>