feat(dx): add make dev one-command local setup

Simplifies local development from 3+ commands to a single `make dev`
that auto-detects environment (main/worktree), creates env files,
installs dependencies, starts PostgreSQL, runs migrations, and launches
both backend and frontend.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/dev.sh

@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+cd "$REPO_ROOT"
+
+# ---------- Check prerequisites ----------
+missing=()
+command -v node >/dev/null 2>&1 || missing+=("node")
+command -v pnpm >/dev/null 2>&1 || missing+=("pnpm")
+command -v go >/dev/null 2>&1 || missing+=("go")
+command -v docker >/dev/null 2>&1 || missing+=("docker")
+
+if [ ${#missing[@]} -gt 0 ]; then
+  echo "✗ Missing prerequisites: ${missing[*]}"
+  echo "  Please install: Node.js v20+, pnpm v10.28+, Go v1.26+, Docker"
+  exit 1
+fi
+
+# ---------- Environment file ----------
+if [ -f .git ]; then
+  # Inside a git worktree (.git is a file, not a directory)
+  ENV_FILE=".env.worktree"
+  if [ ! -f "$ENV_FILE" ]; then
+    echo "==> Worktree detected. Generating $ENV_FILE..."
+    bash scripts/init-worktree-env.sh "$ENV_FILE"
+  fi
+else
+  ENV_FILE=".env"
+  if [ ! -f "$ENV_FILE" ]; then
+    echo "==> Creating $ENV_FILE from .env.example..."
+    cp .env.example "$ENV_FILE"
+  fi
+fi
+
+echo "==> Using $ENV_FILE"
+
+set -a
+# shellcheck disable=SC1090
+. "$ENV_FILE"
+set +a
+
+# ---------- Install dependencies ----------
+if [ ! -d node_modules ]; then
+  echo "==> Installing dependencies..."
+  pnpm install
+fi
+
+# ---------- Database ----------
+bash scripts/ensure-postgres.sh "$ENV_FILE"
+
+echo "==> Running migrations..."
+(cd server && go run ./cmd/migrate up)
+
+# ---------- Start services ----------
+echo ""
+echo "✓ Ready. Starting services..."
+echo "  Backend:  http://localhost:${PORT:-8080}"
+echo "  Frontend: http://localhost:${FRONTEND_PORT:-3000}"
+echo ""
+
+trap 'kill 0' EXIT
+(cd server && go run ./cmd/server) &
+pnpm dev:web &
+wait