fix(migration-to-aws): catch dangling _advances_to (validator false-green)

[icarthick]

What

The frontmatter validator silently passed a phase whose _advances_to names a phase that doesn't exist — a false green on a broken backbone.

Root cause

The backbone chain-consistency block is gated on advTargetsResolvable, which self-skips the moment any _advances_to is unresolvable (it assumes "that phase just isn't frontmatter-annotated yet — partial rollout"). And unlike _requires_phase, _advances_to had no independent membership check. So a genuine dangling/typo'd forward edge disables the very checks meant to catch it.

Reproduced: setting generate._advances_to to a non-existent phase (generate has no _re_entry_guard to incidentally trip) → validator reports OK, exit 0. CI would pass a backbone that dead-ends nowhere. (A typo on discover only went red by luck — its _re_entry_guard._stale_if_completed cross-check tripped a different rule.)

Fix

An independent dangling-forward-edge check: each non-terminal _advances_to target must resolve to a phase directory on disk (references/phases/<name>/<name>.md), not the frontmatter-declared set.

This preserves partial-rollout tolerance — an edge to a real phase whose file exists but has no frontmatter yet still resolves — while failing only on an edge to a phase that doesn't exist at all. (In the real heroku rollout the prose file was always present when frontmatter lagged, so this matches how rollout actually happened.)

Changes

• check.ts — +independent dangling-forward-edge check (disk membership), placed before the rollout-gated chain block so it can't be self-skipped.
• tests — goodSkill() now includes a frontmatter-less clarify.md stub so its partial-rollout test is faithful (real phase, no frontmatter → tolerated, resolves on disk); +1 regression test for a truly-absent target (typo → fail). 41 tests total.

Verification

• mise run build green: lint:md, lint:types (tsc strict), lint:frontmatter (real skill clean), 41/41 tests, fmt:check.
• Manually confirmed: broken generate._advances_to now fails with a clear message; real skill stays clean.

Ownership

Touches only team-owned validator + tests — no admin-owned paths.