feat(pr-convention-policy): record Refs: and negated-closer rules in policy.json #647

Problem

claude-code-plugins#4636 changed the hook validator pr-linkage-validator.sh to recognize non-closing references (Refs: lines, the NON_CLOSING_ERE declaration) and negated closers (for example "does not close #N"). components/pr-convention-policy/policy.json has no field for either rule: its body object carries only closingKeywords, noIssueMarkers and requiredSections.

lockstep-drift.mjs was updated in #634 to parse the new validator shape, but it only checks CLOSING_ERE and NO_ISSUE_ERE against the policy. The Refs: and negated-closer behavior therefore lives in the hook alone, with no canonical record in the policy and no drift check against the ci-workflows pr-contract composite.

Proposal

• Add policy fields for non-closing reference markers and negated-closer handling (bump schemaVersion if the schema requires it).
• Extend lockstep-drift.mjs to extract NON_CLOSING_ERE (and any negation pattern) and assert it matches the new fields.
• Confirm whether the pr-contract composite in ci-workflows implements the same rules; if not, decide whether it should, or record the hook as intentionally stricter.

Acceptance

• policy.json names both rules.
• npm run test:pr-convention-lockstep fails when the hook's NON_CLOSING_ERE drifts from the policy.