Add warn-only governance checks and an expiring maintainer override record #2682

Problem

The accepted Base governance design and implementation plan identify package 19 as a required deliverable. This issue translates that package into a reviewed, bounded issue/PR train; it does not authorize implementation before the issue reaches Ready.

Expected behavior

Implement warn-only lifecycle checks with no merge block, no rollback/reversion, and no automated status mutation. Provide a scoped, reasoned and expiring maintainer override record; agents must not write or approve overrides. Keep automated execution disabled until package 20 pins a released checker.

Scope and non-goals

Scope: Implement warn-only lifecycle checks with no merge block, no rollback/reversion, and no automated status mutation. Provide a scoped, reasoned and expiring maintainer override record; agents must not write or approve overrides. Keep automated execution disabled until package 20 pins a released checker.

Non-goals: Do not silently expand scope, activate unreviewed policy, mirror issue fields onto PRs, grant agents merge/release authority, or make changes outside the named affected areas. Any new product decision or materially new work gets a separate issue and decision record.

Affected areas

• Governance check runner, workflow policy and contributor/maintainer docs.
• Accepted source: governance design and implementation plan, PR #2664
• Parent tracker: #2665

Impact and evidence

1. Product intent: Implements only the accepted governance design and plan; any product-requirement change must be filed separately.
2. Architecture and contracts: Destination is the package-specific section of the merged governance design/implementation plan plus the affected canonical contract named above.
3. User guidance: The issue identifies likely guidance paths; the implementation PR must name updated user/FAQ/README paths or give a change-specific reason why none is affected.
4. CLI/API and reference: The affected-area list determines whether CLI/API/reference docs change; the implementation PR must cite the exact reference path or justify no impact.
5. Operations and safety: No privileged automation or merge authority is implied; the PR must list operational/security effects and any rollback.
6. Validation and testing: The issue lists the planned tests; the PR must report executed validation and explain any unavailable test.
7. Examples and downstream use: The implementation PR must identify affected examples/consumers or give evidence that this change has no downstream surface.
8. Governance and contributor process: This package changes or preserves the contributor/governance contract as described in Scope; links must point to the canonical rules.
9. Decision records: No new decision is accepted by this issue; any durable deviation requires a proposed decision record and explicit maintainer acceptance.
10. Release notes: The PR must include a user-facing release-note line or a reasoned none; this issue does not pre-decide that disposition.

These are issue-stage dispositions to refine during Triage. Before Ready, every category must have its exact destination or evidence-based no-impact reason; the PR repeats the ten-category evidence for actual changes.

Open questions

• None known. If implementation exposes an unresolved contract, return this issue to Triage and record the decision before implementation continues.

Plan

• S1: One focused PR for this issue. Use Fixes #N only if no additional slice is needed; if the maintainer approves multiple PRs, name each S1…Sn here before the first is opened and use Part of #N.
• Dependencies: the P7c issue, the P9 issue
• Acceptance criteria:
• Every rule starts warn-only and reports stable evidence/identifiers without blocking or reverting work.
• Overrides name the maintainer, reason, scope, expiry and rule; agent-produced text cannot grant an override.
• No issue status, branch, PR, release or policy changes are written by this package.
• Docs explain how to review warnings and withdraw/expire an override.
• Validation:
• Fixtures prove warnings do not fail required checks or mutate GitHub state.
• Test valid, expired, malformed and agent-authored override attempts.
• Verify no hosted execution occurs before the pin gate in package 20.
• Risks and rollback: A warning can be mistaken for approval; label it non-authoritative and keep rules disabled or warn-only until promoted with evidence.
• Decision impact: No decision is accepted merely by implementing the issue. Record the relevant accepted decision/ADR and file a superseding record if the maintainer approves a change.