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.

.dockerignore

@@ -0,0 +1,12 @@
+node_modules/
+.cw/
+workdir/
+.git/
+apps/server/dist/
+apps/web/dist/
+coverage/
+*.log
+.env*
+.DS_Store
+.screenshots/
+.cw-previews/