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.

scripts/entrypoint.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+set -e
+
+# 1. Initialize workspace
+mkdir -p /workspace/.cw
+echo '{"version":1}' > /workspace/.cwrc
+
+# 2. Git globals
+git config --global user.email "preview@codewalkers.dev"
+git config --global user.name "Codewalkers Preview"
+git config --global protocol.file.allow always
+git config --global init.defaultBranch main
+
+# 3. Start backend
+cd /workspace
+CW_DB_PATH=/workspace/.cw/cw.db node /app/apps/server/dist/bin/cw.js --server &
+BACKEND_PID=$!
+
+# 4. Wait for health
+echo "Waiting for backend to start..."
+TRIES=0
+MAX_TRIES=60
+until curl -sf http://localhost:3847/health > /dev/null 2>&1; do
+  TRIES=$((TRIES + 1))
+  if [ "$TRIES" -ge "$MAX_TRIES" ]; then
+    echo "Backend failed to start after ${MAX_TRIES}s"
+    exit 1
+  fi
+  sleep 1
+done
+echo "Backend is healthy"
+
+# 5. Run Caddy in foreground (receives signals)
+exec caddy run --config /etc/caddy/Caddyfile