Validate roadmap frontier references against live GitHub state #205

Follow-up from #187

The immediate stale Current Frontier text is corrected by PR #206. This issue owns the optional automated safeguard proposed in #187; it must not turn a documentation correction into an unbounded CI change.

Acceptance

• Define a small explicit frontier reference format and its authority (GitHub issue, PR, milestone, or an explicit no-active-phase state).
• Reject a current frontier that names closed work without a live successor, using deterministic fixtures for the checker.
• Distinguish a stale frontier from unavailable GitHub evidence; authentication or network failure must not silently count as success.
• Define when the network-backed check runs, its permissions, and its failure policy. Keep offline compiler/validation paths deterministic.
• Add RED/GREEN evidence and update the owning workflow/documentation topic before claiming enforcement.

No software enforcement exists merely because the roadmap prose has been corrected. Avoid testing for a prose phrase or filename as a substitute for checking external state.