feat(skills): add deferred searchable availability

[andrei-hasna]

Summary

• model catalog availability as Enabled, Deferred, or Disabled
• omit deferred skills from ambient prompt rendering while keeping them explicitly searchable and loadable
• keep disabled skills fail-closed and add host-policy plus behavior regressions

Verification

• just test-fast -p codex-skills-extension (4 passed)
• just fix -p codex-skills-extension
• just fmt
• git diff --check
• pre-commit and pre-push Todos redaction scans
• independent adversarial review: no P0/P1 blockers

Integration note

OPE2-00068 should gate future model-facing provider search/read routing with is_searchable and is_explicitly_loadable.

Todos: OPE2-00066