PRD: model capability by job, not named-model review

[cahenesy]

Drop the requirement that review run on a different named model.
Independence is a fresh worker. Judgment work defaults to the most
capable model; the only cheap slot is mechanical runtime-verify.

Open assumptions & waivers:

• review independence is worker/session not model-name — resolved: independent worker remains a flip requirement; same named model is allowed for write and review
• cheap-model slot is FR-52 mechanical verify only — resolved: parent sequencer stays most capable
• authoring sessions warn when not most capable — resolved: /prd-author, /tdd-author, and /build-tdds warn and ask continue/stop when the parent session is below the latest top-tier binding or the session model cannot be read
• cheaper write/review pin is warned not refused — resolved: honor THROUGHLINE_BUILD_MODEL / THROUGHLINE_REVIEW_MODEL with a warning; run continues
• most capable is the latest top-tier binding — resolved: one binding site; rebinding on a new generation is an implementation change
• design-reviewer is most capable independent worker — resolved: FR-10 is a fresh worker on the most-capable default; same-name as the TDD author is allowed
• session model is always observable on supported harnesses — resolved: Claude Code and Grok Build expose it; unreadable is a defect with the same warn-and-ask shape
• /build-tdds parent session is also warn+ask — resolved: same check as authoring

Cascade:

• ADR 0009 (accepted) — supersede. Its decision (review = prior-gen top tier to keep author≠reviewer by name) is no longer the requirement.
• ADR 0008 (accepted) — refine or supersede remaining author≠reviewer-by-name consequence. Rework authoring on the build model is already vacated by ADR 0013; the diversity rationale is what this PRD change removes.
• ADR 0010 (accepted) — refine the consequence "Review flip authority is one independent different-model artifact" to independent worker, not different-model.
• TDD 0057 (implemented) — supersede. It bound review to prior-gen top tier.
• TDD 0062 (implemented) — supersede the models.sh / NFR-3 pairing (review = prior-gen). New TDD(s) must rebind review (and design-reviewer, judgment verify) to the latest top-tier default, add the cheaper-pin warning (FR-87), and add the parent-session warn-and-ask (FR-86) on /prd-author, /tdd-author, and /build-tdds.
• Skills and README still say "different model" / prior-gen review (skills/implement, skills/tdd-author, agents/design-reviewer.md, README.md). Those are implementation follow-through for /tdd-author, not this PR.