docs(backlog): fix stale WP-19 CI description; add per-step model guidance

WP-19: the Files section still described the originally-planned CI approach
(background servers with & + wait-on); the actual implementation (documented in
ci.yml's own comment and playwright.config.ts) moved server lifecycle into
Playwright's webServer to fix the 2-hour e2e hang. Recorded as a Deviation entry
and corrected the Files section to match reality. Spot-checked the rest of the WP
(retryClicked wiring, the currentScenario() claim, the still-open retry gaps in
registration-detail/aanvraag-detail/brief) against current code — all still hold.

Session protocol: replace the removed model mandate with per-step guidance —
Opus for planning, Sonnet for implementation, Haiku subagents for simple
read-only CLI checks — so cost matches the step's difficulty, not the whole WP.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/project/backlog/README.md

@@ -11,6 +11,12 @@ This backlog **supersedes `docs/project/SHOWCASE-ROADMAP.md`**.
 
 - **One WP per session.** Read `CLAUDE.md`, this README, the WP file, and the WP's
   "Read first" list — then execute. Do not start the next WP in the same session.
+- **Match the model to the step, not the whole session.** Plan/design under Opus (`/model
+opus`) — a WP's approach and edge cases deserve the stronger model. Switch to Sonnet to
+  write the code once the plan is approved — routine implementation against a pre-made
+  Decisions block doesn't need Opus. Delegate simple, read-only CLI checks (running a test
+  suite, `grep`/`git status`, verifying a file exists) to a Haiku subagent where the harness
+  supports it — proportion cost to the step's difficulty, not the WP's.
 - The **Decisions** block in each WP is pre-made — don't relitigate it.
 - A WP ends **GREEN** (below) with its acceptance criteria checked off and its Status
   updated to `done` (+ commit hash).