feat(team): enforce fail-closed Skill guard checks

[TyreeZhao]

Summary

• Integrate the Skill guard client into admin Skill content writes and extension package imports.
• Persist pending scan state and poll the same task until a terminal state, with fail-closed behavior for pending, rejected, unavailable, and timed-out results.
• Make pending scan recovery idempotent across approval, cancellation, persistence failure, and retry paths.
• Keep raw Skill content, uploaded archives, credentials, and full guard responses out of audit and debug output.
• Constrain extension rule operations to global rules and include the baseline test-stub compatibility fix required by the current main branch.

Verification

• go test ./... under backend/
• Targeted guard, audit, config, team repository, and team usecase tests
• git diff --check