[P2] Add artifact-level deprecation

[spazyCZ]

Objective

Define whether individual skills, commands, or agents can be deprecated independently of the package.

Priority

P2 — Nice to Have

Details

The spec currently supports package-level deprecation, but not artifact-level deprecation within a package.

Acceptance Criteria

• Decide whether artifact-level deprecation is in scope
• If yes, define metadata shape and behavior
• Clarify UI, registry, and migration expectations

Notes

Source: work/spec-gap-analysis.md