chore(scripts): add parallel build scripts

Replace sequential `npm run build --workspaces --if-present` with
concurrent execution via a reusable `parallelTask` utility. Full
`npm run build` now runs cleanup then all 7 build tasks in parallel.

scripts/build.js

@@ -0,0 +1,45 @@
+#!/usr/bin/env node
+/**
+ * Full build pipeline. Equivalent to:
+ *   npm run cleanup && npm run build --workspaces --if-present
+ *                    && npm run build:website -w @page-agent/website
+ *                    && npm run zip -w @page-agent/ext
+ *
+ * 1. cleanup
+ * 2. build everything in parallel (libs + website + extension)
+ */
+import chalk from 'chalk'
+import { execSync } from 'child_process'
+import { readFileSync } from 'fs'
+import { dirname, join } from 'path'
+import { fileURLToPath } from 'url'
+
+import { parallelTask } from './parallel-task.js'
+
+const rootDir = join(dirname(fileURLToPath(import.meta.url)), '..')
+const rootPkg = JSON.parse(readFileSync(join(rootDir, 'package.json'), 'utf-8'))
+
+// Step 1: cleanup
+console.log(chalk.bgBlue.white.bold(' ▸ cleanup '))
+execSync('npm run cleanup', { cwd: rootDir, stdio: 'inherit' })
+
+// Step 2: build all in parallel
+console.log(chalk.bgBlue.white.bold(' ▸ build '))
+const tasks = rootPkg.workspaces
+	.map((ws) => {
+		const dir = join(rootDir, ws)
+		const pkg = JSON.parse(readFileSync(join(dir, 'package.json'), 'utf-8'))
+		return pkg.scripts?.build ? { label: pkg.name, command: 'npm run build', cwd: dir } : null
+	})
+	.filter(Boolean)
+
+tasks.push(
+	{
+		label: '@page-agent/website',
+		command: 'npm run build:website',
+		cwd: join(rootDir, 'packages/website'),
+	},
+	{ label: '@page-agent/ext', command: 'npm run zip', cwd: join(rootDir, 'packages/extension') }
+)
+
+await parallelTask(tasks, { timeoutMs: 120_000 })