feat(mgtp): DECISION_ARTEFACT_V0_1 – deterministic, time-bounded, evidenced decision record

[Copilot]

Introduces a canonical, replayable DecisionRecord artefact for MGTP with fail-closed time-window and evidence enforcement.

mgtp/types.py

• Added Verdict(str, Enum) — ALLOW | REFUSE | SUPERVISED
• Added EvidenceRef frozen dataclass — ref_id + description

mgtp/decision_record.py (new)

• DecisionRecord frozen dataclass — fields: record_id, decision_time, authority window [start, end], verdict, reason_codes (sorted tuple), provided_evidence (sorted tuple or None)
• canonical_bytes() — deterministic UTF-8 JSON: sort_keys=True, compact separators, reason codes and evidence refs pre-sorted; explicit field set via _CANONICAL_FIELDS tuple
• decision_hash property — sha256(canonical_bytes()) lowercase hex
• evaluate() — constructs a DecisionRecord after applying two fail-closed rules in order:
  1. decision_time ∉ [window_start, window_end] → force REFUSE + append outside_authority_window
  2. verdict == ALLOW and provided_evidence is None → force REFUSE + append evidence_missing

rec = evaluate(
    record_id="rec-001",
    decision_time="2026-02-28T12:00:00Z",
    authority_window_start="2026-02-28T00:00:00Z",
    authority_window_end="2026-02-28T23:59:59Z",
    requested_verdict="ALLOW",
    reason_codes=["allowlist_match"],
    provided_evidence=[EvidenceRef(ref_id="ev-001", description="owner confirmed")],
)
# rec.decision_hash == sha256(rec.canonical_bytes())  — stable across processes

tests/fixtures/golden_decision.json (new)

Frozen base64 canonical bytes + sha256 hash for cross-process replay regression.

tests/test_decision_record_determinism.py (new)

12 tests covering:

• Determinism — canonical_bytes() and decision_hash identical across N=50 calls
• Golden replay — exact byte/hash match against stored fixture
• Time-bound — before/after window → REFUSE; inside window → unchanged
• Evidence — ALLOW without evidence → REFUSE + reason code; REFUSE/SUPERVISED without evidence → unchanged
• Surface-area guard — SHA-256 file-hash assertions on authority_gate.py, stop_machine.py, and all commit_gate/** Python files to detect accidental no-touch violations

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