Commit action authority mode to independent checkpoint v3

[aruintelligence]

Change

• New file authority checkpoints use protocol 3, extending the independently retained state digest to cover authority-mode.json alongside decision files.
• A valid local mode downgrade now disagrees with the external checkpoint before issuance or callback. The check repeats after an awaited external claim.
• Protocols 1 and 2 retain their original read and write semantics; no in-place upgrade occurs. The tests include a protocol-2 counterexample showing the earlier mode gap and a protocol-3 rejection.
• Update the checkpoint contract and project documentation with the migration and threat boundaries.

Verification

• npm test — 1,014 passed
• node scripts/check-package-artifact.mjs — 42 checks passed
• npm run compatibility:consumer — passed
• npm run check:links — 411 Markdown files passed

Limits

The included file anchor is a one-machine example under the same OS identity. Protocol 3 depends on an independently retained external checkpoint; restoration or modification of both stores, tool paths that bypass this authority, and provider acceptance remain outside this proof. Existing protocol-1/2 stores do not silently gain the new binding.