docs(questionnaires): F9.3 forking documentation

[JohnD-EE]

What & why

F9.3 — Forking documentation, the fork-readiness deliverable of phase P9. Two readers need a map: a client-engagement team inheriting this repo to deliver a different vertical, and any fork tracking upstream Sunrise. This PR writes both maps and records the upstream contributions ConQuest's own build already drove.

Docs-only — no code, schema, or route changes.

Deliverables

• .context/app/questionnaire/forking.md (new, keystone) — two halves:
  • Part A — Re-skinning for a new vertical: what to keep verbatim (the platform + the vertical-neutral questionnaire engine), the rename-surface table (18 App* models, 18 app_* tables, 11 APP_QUESTIONNAIRES_* flags, live questionnaire/ConQuest counts), ordered sed recipes with the app_questionnaire-before-questionnaire collision rule and the "don't touch the lib/app/ bootstrap seams" guard, what content to replace, and the demo-tenancy replacement (grep DEMO-ONLY; three paths — delete / AppTenant+RLS / AppBrand).
  • Part B — Tracking upstream Sunrise: wiring the upstream remote, the merge recipe, semantic-conflict discipline (never edit Sunrise's migration SQL), version-decision references, and the contribute-back loop.
• .context/app/planning/contributions-upstream.md (new) — resolved-counterpart ledger to upstream-gaps.md: UG-1/UG-2 table (Sunrise PR #286/#285, both in v0.0.1) with honest "smaller than planned, still a real sales artefact" framing.
• .context/app/planning/features/f9.3.md (new) — feature tracker, modeled on f9.4.md.
• Registered forking.md in the questionnaire docs index; flipped the three F9.3 status lines in development-plan.md to done.

Design notes

• Cross-reference, don't duplicate or edit platform docs. CUSTOMIZATION.md, VERSIONING.md, .context/database/migrations.md are platform-owned (they merge through from upstream) — they are linked as authoritative, never edited. The new value is ConQuest-specific detail and the sequencing of the upstream-tracking mechanics into one guide.
• forking.md is the canonical home for the three demo-tenancy replacement paths — demo-clients.md § "Fork guidance" already links to it for them.
• No CHANGELOG.md entry — app-surface docs, not Sunrise platform public surface (per the repo's platform-scoped CHANGELOG policy).

Verification

• All cross-reference links in the three new docs resolve to real files; cited npm scripts (db:drift-check, db:reset, …) and the Sunrise upstream URL check out.
• Rename-surface counts and the 18-model / 11-flag inventory spot-checked against the repo.
• Prettier-clean (lint-staged ran on commit).
• The sed recipes are a documented procedure — not run against the tree.

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