Make the repo legible to someone seeing it for the first time

The README claimed screenshots were "coming" and stopped describing the
project around phase 10, so the world-state engine, the retry/undo state
revert and the egress work — the most interesting parts — were invisible.

- Screenshots of the play screen, Insights, the schema editor and the
  script editor, captured from the running app.
- Document the world-state engine, undo/retry rewind, and the two measured
  performance fixes; correct the context assembly order to match
  context/builder.py; drop the stale "phases 1-6 complete" note.
- Add CI (backend pytest, frontend lint + build, Docker image build) and
  its badge. Nothing ran the 151 tests but me.
- Move CODE_REVIEW_FINDINGS.md to docs/self-review.md and say up front
  that every correctness finding is resolved.
- Drop a dead import that the newly-wired lint flagged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015sygnUWH8WnaoKPDp7hXM1

.github/workflows/ci.yml

@@ -0,0 +1,74 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  backend:
+    name: Backend tests
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"   # matches the Dockerfile
+          cache: pip
+          cache-dependency-path: |
+            backend/requirements.txt
+            backend/requirements-dev.txt
+
+      - name: Install dependencies
+        working-directory: backend
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt -r requirements-dev.txt
+
+      - name: Run tests
+        working-directory: backend
+        run: python -m pytest tests/ -q
+
+  frontend:
+    name: Frontend lint + build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: "24"       # matches the Dockerfile
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install dependencies
+        working-directory: frontend
+        run: npm ci
+
+      - name: Lint
+        working-directory: frontend
+        run: npm run lint
+
+      - name: Build
+        working-directory: frontend
+        run: npm run build
+
+  docker:
+    name: Docker image builds
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: docker/setup-buildx-action@v3
+      - name: Build image
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: false
+          cache-from: type=gha
+          cache-to: type=gha,mode=max