docs: define archive skill transport policy

[akiezun]

Summary

• records the verified 51-skill archive inventory and clean-room reuse policy
• assigns every skill one disposition, target layer, and replacement owner
• defines dependency-aware PR boundaries for future transport work
• adds tests binding the human plan to the machine-readable manifest

Scope

This is P00 from ARCHIVE_SKILL_TRANSPORT_PLAN.md. The 1,203-line diff exceeds the normal warning threshold because it contains the complete 51-row review, the downstream PR contract, and its machine manifest; it adds no runtime behavior or generated assets.

Non-goals

• no archived skill implementation
• no autonomous execution on main
• no model/provider integration
• no archive redistribution or verbatim reuse
• no PR merges

Dependencies

None. Later P-series work depends on this policy.

Validation

• make lint
• make test: 542 passed, 1 skipped
• make check-example
• plugin validator: passed
• git diff --check

Risk and migration

Documentation, policy data, and tests only. Existing plugin and CLI surfaces are unchanged.

Rollback

Revert commit ccf1302; no runtime or persisted project data is affected.