docs(plan): P1 prod write-side implementation plan (7 tasks, TDD on nginx template)

- docs/plans/2026-09-29-prod-write-side.md: crearte nginx template ->
  crearte-deploy compose/.env/README -> four-profile config gate -> prod
  live run + curl smoke chain (write side + wildcard subdomain runtime)
  -> merges with --no-ff -> wrapper bookkeeping
- docs/ROADMAP.md: register plan in doc index
- facts pinned from recon: no .env/no crearte volumes on this box (fresh
  env), :? guards are file-wide (not per-profile), curl needs --resolve
  for *.localhost, set-role invalidates tokens (re-login required)

docs/ROADMAP.md

@@ -51,4 +51,4 @@
 | 子项目 | spec | 实现计划 |
 |--------|------|----------|
 | P4 作者主页 | `docs/specs/2026-09-29-author-page-design.md` | `docs/plans/2026-09-29-author-page.md`（已执行，2026-09-29） |
-| P1 prod 写侧可用 | `docs/specs/2026-09-29-prod-write-side-design.md` | 待撰写（spec 审阅通过后） |
+| P1 prod 写侧可用 | `docs/specs/2026-09-29-prod-write-side-design.md` | `docs/plans/2026-09-29-prod-write-side.md`（待执行） |