Add contextual append claim help

[anusbutt]

Summary

• add repository-independent contextual help for append and append claim
• derive claim flag acceptance and displayed help from one shared catalogue
• direct agents to installed CLI help instead of package-source inspection
• cover exact dispatch, no-mutation behavior, packaged installs, skill guidance, and public docs

Verification

• npm run typecheck
• npm run validator:check
• npm test
• npm run package:smoke
• fresh non-repository quickstart
• Benchmark B: 3/3 sessions used append claim help and 0/3 inspected installed source for claim syntax