PRD: /ux-author phase for UI mocks between PRD and design

[cahenesy]

Adds an optional /ux-author phase between /prd-author and /tdd-author.
/prd-author marks UI-bearing requirements with [UI] (FR-89). For those,
/ux-author produces an in-repo UX set under docs/ux/ (FR-90-FR-100). The
set holds HTML mocks per screen and state, a flow index, a UX index that
maps screens to FR ids, and PNG screenshots when a renderer is available.
An independent fresh-worker critique runs, then a docs/ux/ PR that
the human merges. The design work is delegated to the design skills and
tools on the harness. With none present, /ux-author degrades and declares
the fidelity it reached. /tdd-author designs [UI] requirements against
merged mocks and refuses un-mocked ones unless the user waives them with
a rationale (FR-101). Mocks are design input, not a build gate.

Open assumptions & waivers

• Hi-fi bar vs degraded mode — resolved: hi-fi applies when a design delegate is present; degraded runs declare the fidelity reached, and the critique judges against it (FR-92).
• Screenshot rendering without a headless browser — resolved: commit HTML only; the PR states "screenshots not rendered: "; not a failure (FR-91).
• Greenfield design system — resolved: an ADR via /adr-new in the UX PR plus a committed tokens artifact that later screens and TDDs honor (FR-93).
• Mobile mocks in HTML — resolved: HTML at named device viewports following iOS/Android conventions; native rendering is a non-goal (FR-91).
• Mock drift over time — resolved: a point-in-time record per PRD revision, superseded by later runs, not kept in sync with shipped UI (FR-99).
• Grok design ecosystem — resolved: same contract on both harnesses, may run degraded on Grok; the delegate inventory is an open question for /tdd-author.
• Brownfield current UI — resolved: baseline derived from code, plus an optional user-supplied live URL; the index records the baseline per screen (FR-94).
• Delegate provenance — resolved: the committed UX index records the delegates used and the declared fidelity; the PR body repeats both (FR-92).
• PNG repo weight — resolved: commit PNGs normally; a superseding run removes superseded screenshots; no LFS (FR-99).
• Authoring machinery for /ux-author — resolved: drafts/resume (FR-46-49), interrogator (FR-75), rubric (FR-77), and the FR-86 check all apply (FR-100).
• Live-URL capture privacy — resolved: captures are reference-only for the run and never committed (FR-94).

Cascade

Additive: no existing TDD becomes stale in its own scope, but these
skills and requirements need design work:

• /prd-author (FR-89): ask about and write the [UI] marker. This revises
  the /prd-author design (implemented TDDs, so supersede the affected
  part rather than edit it).
• /tdd-author (FR-101): find merged UX sets, refuse un-mocked [UI]
  requirements, record waivers, and cite screens in the traceability
  table. Same supersession note.
• New skill /ux-author plus a UX critique worker (FR-90-FR-100). This
  reuses the draft helpers (FR-46-49), the FR-86 check, the rubric and
  interrogator patterns, and /adr-new.
• Enumerations amended in this PR: FR-79, FR-86, FR-87, FR-22
  acceptance, NFR-1, NFR-3. Enumerations still naming only
  /prd-author and /tdd-author, now covered by FR-100: the FR-46-49
  section heading and text, FR-75/76, FR-77 ("Both").
• ADRs: no accepted ADR is contradicted. ADR 0001 (throughline owns
  the authoring phases, delegates ideation) is extended to a third
  authoring phase; consider whether /tdd-author records that in an
  ADR. FR-93 may create a per-project design-system ADR in consumer
  repos.

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