Stage 16: insert Stage 17 (test-contour verification); renumber prod deploy to 18

- PLAN.md: new Stage 17 "Test-contour verification & defect fixes" (exercise the
  deployed contour end-to-end and fix what it surfaces — connector liveness check,
  path-conditional CI); the former prod-deploy stage becomes Stage 18.
- Renumber every "Stage 17" prod-deploy reference to "Stage 18" across docs,
  compose, Caddyfile, ci.yaml and CLAUDE.md; the post-Stage-14 split range is now
  "Stages 15–18".

CLAUDE.md

@@ -61,7 +61,7 @@ conversation memory — is the source of continuity. Keep it that way.
   (`docker compose up -d --build` on the runner host + a `GET /` probe). A PR into
   `master` is test-only.
 - Merge `development → master` only when CI is green; the **prod** deploy is then a
-  **manual** workflow (Stage 17), never automatic. Secrets/variables are prefixed
+  **manual** workflow (Stage 18), never automatic. Secrets/variables are prefixed
   `TEST_` / `PROD_` per contour (Gitea 1.26 has no deployment environments).
 - After any push, watch the run to green before declaring a stage done — use the
   ready-made watcher, never an inline poll loop: