PRD: model cost/performance by job (inherit parent, escalate on demand)

[cahenesy]

The bar for model choice becomes best cost-to-performance per job, not
the most capable model. Judgment workers inherit the parent session's
model and effort; mechanical runtime-verify runs on a light tier at low
effort, never more expensive than the parent; /build-tdds can escalate a
TDD's judgment workers to the most capable model (operator request, or
automatically on a resumed gate FAIL with the TDD unrevised), falling
back to the parent model with a warning when the escalation target is
unusable. FR-86 narrows to a light-tier/unreadable parent check with no
web fetch. Rewrites NFR-3, FR-52, FR-86, FR-87; adds FR-88; updates
FR-10/FR-15 reviewer wording, non-goals, constraints, open questions,
and the evaluation rubric.

Open assumptions & waivers:

• newer is not more capable — resolved: escalation targets the most capable model available, not the newest.
• FR-86 conflicts with cost/perf — resolved: narrowed to light-tier or unreadable parent only.
• auto-escalation pays for spec defects — resolved: escalate only when the TDD is unrevised since the halt.
• no machine-readable capability ranking — resolved: maintained binding + env override, lag accepted.
• expensive parent silently inherited — resolved: accepted as operator choice; queue confirmation names each worker's model before dispatch.
• harness may not support per-worker model — resolved: treated as unavailable: warn, record fell-back, run on parent.
• escalation no-op when parent is top — resolved: run record states escalation=already-top.

Cascade:

• TDD 0064 (draft) — STALE, needs revision: binds unset build=review=fable
  (most capable by default) and a cheaper-pin warning against that default;
  NFR-3/FR-87 now require inheriting the parent and a light-pin warning.
  Its halted build branch (build/20260816-174909/0064-judgment-worker-defaults)
  was built to the old requirement and should be abandoned, not resumed.
• TDD 0065 (draft) — STALE, needs revision: FR-86 no longer compares to the
  most capable model or fetches the models page; it is a light-tier /
  unreadable-parent check.
• TDD 0062 (implemented) — pressured: introduced scripts/lib/models.sh with
  pinned judgment defaults; the new design changes that binding site's
  contract (supersede the model-binding part).
• New design needed for FR-88 (escalation trigger, outcomes, fallback
  detection) and the effort inheritance/pins in FR-87/FR-52.
• ADR 0014 (accepted) — needs supersession: its decision binds unset
  build/review to the latest top-tier id and FR-86 to a live models-page
  fetch. Its fresh-worker independence decision carries forward.
• ADR 0008 / 0010 — unaffected.

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https://claude.ai/code/session_01Ki3TDnP2bNqqpEkFoZaEXq