skills/design-issue: audit and reconcile pre-existing child sub-issue titles and bodies against design decisions in design:stale-bodies

[JoshuaVSherman]

Part of #737

What this builds

Enhances src/design-issue/stale_bodies.ts and deno task design:stale-bodies so it audits not only parent Epic bodies, but also all child sub-issues listed in the Gate 2 plan table or attached to the Epic against the latest decisions, endpoints, and requirements in the design document:

1. Child Sub-Issue Drift Detection: Reads child sub-issue references from the design document plan table / Epic and fetches their remote issue bodies via gh issue view.
2. Decision & Verb Reconciliation: Compares sub-issue titles and acceptance criteria against the design document's Decisions Record and Architecture sections (e.g. flagging when an issue titled for GET is missing a newly added DELETE verb or takedown criteria approved during Gate 0).
3. Actionable Reporting: Reports stale sub-issue titles, outdated acceptance criteria, or missing HTTP methods with clear reconciliation guidance before Gate 2 plan approval.
4. Unit Test Coverage: Adds test cases in test/design_issue_stale_bodies.test.ts covering child sub-issue drift detection and clean passes.

Acceptance criteria

1. Running deno task design:stale-bodies <doc.md> detects and reports pre-existing child sub-issues whose title or body fails to reflect decisions in the design document.
2. When all child sub-issue titles and bodies match the design document's decisions, design:stale-bodies reports zero sub-issue violations.
3. Automated unit tests in test/design_issue_stale_bodies.test.ts pass 100% green.