fix(plugin-sdk): make require('trek-plugin-sdk') actually resolve everywhere

A freshly scaffolded plugin could not load anywhere: the npm package is
ESM-only (no require condition in its exports map), so the scaffold's
require('trek-plugin-sdk') threw ERR_PACKAGE_PATH_NOT_EXPORTED under
`trek-plugin dev` - and the runtime injection the wiki promised for the
plugin child never existed, so a packed plugin (node_modules stripped)
crashed with MODULE_NOT_FOUND after a real install.

- plugin child: inject a frozen {definePlugin, PLUGIN_API_VERSION} shim
  for require('trek-plugin-sdk'); subpaths fail with a pointed error
- trek-plugin dev: inject the exact same shim, so a fresh scaffold runs
  with zero npm install and dev parity with production holds
- npm package: ship a real CommonJS build (dist/cjs + require export
  conditions) so the installed package also requires cleanly on Node 18+
- create: scaffold a package.json (type commonjs, SDK as devDependency,
  npx scripts); print resolvable `npx trek-plugin-sdk ...` hints
- wiki: package.json in the scaffold tree + publishing checklist, and
  document the zero-install dev flow

plugin-sdk/scripts/finish-cjs.mjs

@@ -0,0 +1,9 @@
+// The package root says `"type": "module"`, so the CommonJS second build in
+// dist/cjs needs its own scope marker or Node would parse those files as ESM.
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const dir = path.join(path.dirname(fileURLToPath(import.meta.url)), '..', 'dist', 'cjs');
+fs.mkdirSync(dir, { recursive: true });
+fs.writeFileSync(path.join(dir, 'package.json'), '{"type":"commonjs"}\n');