Add enforcement-grade verification for MGTP determinism and DecisionRecord surface isolation

[Copilot]

Adds verification layer for MGTP: golden determinism fixtures, fail-closed guard enforcement, cross-run stability, and surface isolation checks ensuring MGTP consumes—not redefines—authority contracts. Also adds a CI drift guard preventing modification of forbidden files in PRs.

mgtp/types.py — DecisionRecord artefact

New frozen dataclass with deterministic serialisation:

record = DecisionRecord(
    transition_id="txn-001",
    verdict=TransitionOutcome.APPROVED,
    reasons=("evidence_sufficient",),
    decision_time="2026-01-01T00:00:00Z",
    authority_basis="OWNER",
)
record.canonical_bytes()  # UTF-8 JSON, sorted keys, no whitespace
record.decision_hash      # SHA-256 hex of canonical_bytes()

mgtp/evaluator.py — Fail-closed verdict resolver

Consumes AuthorityGate directly; does not shadow it. Two hard refusals:

• provided_evidence is None → REFUSED / reason: missing_evidence
• decision_time outside [request.timestamp, +1h] → REFUSED / reason: decision_time_outside_authority_window

Irreversible or trust-boundary-crossing requests are always SUPERVISED.

tests/test_mgtp_enforcement.py — 17 enforcement tests

• T1 Golden fixture: pre-computed base64 canonical bytes + SHA-256 hash asserted byte-exact
• T2 Stability: 100-iteration loop on canonical_bytes() and decision_hash
• T3 Fail-closed guards with explicit reason code assertions
• T4 Surface isolation: authority_gate module not mutated post-import; mgtp carries no parallel Evidence/Decision/AuthorityGate definitions

.github/workflows/ci.yml — Forbidden-file drift guard

New forbidden-file-drift-guard job fails any PR that touches authority_gate.py, stop_machine.py, or commit_gate/**. Permissions scoped to contents: read.

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