feat(migrate): add @choojs/migrate — the choo-migrate v7→v8 codemod

Regex-based on purpose: converts simple top-level CJS patterns to ESM,
remaps package specifiers to their @choojs homes, points retired nano*
packages at built-in replacements (choo-lazy-route → lazy()), and reports
everything it refuses to guess at instead of guessing. Validated against
choo's own v7 example app: migrated output runs on v8 unmodified.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014NgfSjHE11oFpoSnLVKLXd

packages/migrate/cli.js

@@ -0,0 +1,52 @@
+#!/usr/bin/env node
+// choo-migrate [--dry] <files or directories…>
+// Rewrites choo v7 sources toward v8: CJS → ESM for simple top-level
+// patterns, package specifiers remapped, and a per-file report of what
+// needs human hands. --dry prints the report without writing.
+
+import { readFile, writeFile, readdir, stat } from 'node:fs/promises'
+import { join, extname } from 'node:path'
+
+import transform from './lib/transform.js'
+
+const args = process.argv.slice(2)
+const dry = args.includes('--dry')
+const targets = args.filter((a) => a !== '--dry')
+
+if (targets.length === 0) {
+  console.log('usage: choo-migrate [--dry] <files or directories…>')
+  process.exit(1)
+}
+
+const SKIP = new Set(['node_modules', '.git', 'dist', 'coverage'])
+
+async function * walk (path) {
+  const info = await stat(path)
+  if (info.isFile()) {
+    if (extname(path) === '.js' || extname(path) === '.mjs') yield path
+    return
+  }
+  for (const entry of await readdir(path)) {
+    if (SKIP.has(entry)) continue
+    yield * walk(join(path, entry))
+  }
+}
+
+let changedCount = 0
+for (const target of targets) {
+  for await (const file of walk(target)) {
+    const source = await readFile(file, 'utf8')
+    const { code, changed, notes } = transform(source)
+    if (!changed && notes.length === 0) continue
+
+    console.log(`\n${file}${changed ? (dry ? ' (would change)' : ' (rewritten)') : ''}`)
+    for (const note of notes) console.log(`  ⚠ ${note}`)
+
+    if (changed && !dry) {
+      await writeFile(file, code)
+      changedCount++
+    }
+  }
+}
+
+console.log(`\n${dry ? 'dry run — no files written' : changedCount + ' file(s) rewritten'}. Remember: package.json needs "type": "module" and deps on @choojs/*.`)