[AIAA] Build step 8: phase-advance workflow

Build step 8 (spec section 16, PR #363):
`.github/workflows/assessment-phase.yml`, the merge-triggered lifecycle
bookkeeping (section 12). Logic lives in a unit-tested script; the
workflow stays thin glue.

Done when unit tests cover:

- [ ] Opening the next phase's tracking issue, linked and unassigned,
on the default route and on the phase B skip route.
- [ ] Posting the transition on the intake issue and swapping its
phase label.
- [ ] Closing the intake on the final phase's merge.
- [ ] With two assessments in flight, every action lands on the intake
the merged PR links, never the other.

Live wiring is verified on the pilot's first phase transition, with
manual bookkeeping as the fallback (HC-4); recorded here.