feat(planning): mark every direct dependent when an interview answer changes or is reopened #9714

Parent

Refs #5796

Depends on

• feat(planning): gate interview page answers on open prerequisites and expect declared dependencies #9709

Outcome

A changed or reopened answer reaches every question that depends on it, not only decided direct dependents
(gap G3):

• When a counted decision changes value or is reopened, every direct dependent becomes stale (it holds a
  decision) or a new upstream-changed state (it does not). An upstream-changed question is not Accept-all
  eligible.
• Re-answering a stale question with a different value marks its own direct dependents the same way: one hop
  per value-changing answer.
• reopen marks dependents like a changed value.
• upstream-changed holds until Claude revises the dependent or posts a reconfirm op; under frontierLock
confirm or hard, Save stays locked until then. Each dependent, not only the parent, shows that the
  upstream change is not handled yet.

Decisions

• The surface's deferred list parks "the stale cascade (automatic transitive re-staling)". This slice keeps
  that parked: propagation moves one hop only when a re-answer changes value. Update that deferred row to say
  the one-hop form is built and the automatic cascade stays parked.
• G1 (owner, 2026-10-18): frontierLock default confirm (shipped by the blocker).

Key interfaces

• Interview page: plugins/planning/surface/index.html; page server: plugins/planning/surface/server.py
  (record(), derive_states); round CLI: plugins/planning/surface/round.py behind round.sh;
  exporters: plugins/planning/surface/exporters.py.
• The interview skill's surface contract, plugins/planning/skills/interview/context/surface.md, documents
  every new op, field, setting and refusal.
• The surface's deferred list, plugins/planning/surface/deferred.md.

Done when

• Server tests: a changed value stales a decided dependent and marks an undecided one upstream-changed; a reopen does the same; a re-answer with a different value on a stale question marks its dependents; a re-answer with the same value marks nothing.
• A server test shows a reconfirm op clears upstream-changed, and Accept-all eligibility excludes upstream-changed questions.
• grep -n "upstream-changed" plugins/planning/skills/interview/context/surface.md and grep -n "reconfirm" plugins/planning/skills/interview/context/surface.md print lines.
• grep -n -i "stale cascade" plugins/planning/surface/deferred.md prints a row that names the one-hop form as built.
• scripts/run-test-file.sh plugins/planning/surface/test_server.py exits 0.
• scripts/run-test-file.sh plugins/planning/surface/test_round.py exits 0.
• bash plugins/planning/surface/surface.test.sh exits 0, so a data dir without the new fields still loads, renders and exports.
• scripts/check-changelog-fragments.sh --check-required origin/main exits 0.

Out of scope

• An automatic transitive cascade (stays parked).
• Stale marking from a new constraint (planning: mark interview recommendations stale when a new constraint arrives mid-interview #5769).