feat: Add Dockerfile, preview config, and seed script for self-preview deployments

Containerize Codewalkers with a multi-stage Docker build (Node + Caddy) and
add a seed script that populates the database with a demo initiative, 3 phases,
9 tasks, 3 agents with JSONL log output, a root page, review comments, and a
git repo with real branch diffs for the review tab.

.cw-preview.yml

@@ -0,0 +1,11 @@
+version: 1
+services:
+  app:
+    build: .
+    port: 3000
+    healthcheck:
+      path: /health
+      interval: 3s
+      retries: 30
+    seed:
+      - sh /app/scripts/seed-preview.sh