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/package.json

@@ -1,6 +1,6 @@
 {
   "name": "trek-plugin-sdk",
-  "version": "1.2.0",
+  "version": "1.2.1",
   "description": "SDK for building TREK plugins: types, definePlugin, a mock host, manifest validation, and the create-trek-plugin / trek-plugin CLIs.",
   "type": "module",
   "license": "MIT",
@@ -13,8 +13,14 @@
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",
   "exports": {
-    ".": { "types": "./dist/index.d.ts", "import": "./dist/index.js" },
-    "./testing": { "types": "./dist/mock-host.d.ts", "import": "./dist/mock-host.js" }
+    ".": {
+      "import": { "types": "./dist/index.d.ts", "default": "./dist/index.js" },
+      "require": { "types": "./dist/cjs/index.d.ts", "default": "./dist/cjs/index.js" }
+    },
+    "./testing": {
+      "import": { "types": "./dist/mock-host.d.ts", "default": "./dist/mock-host.js" },
+      "require": { "types": "./dist/cjs/mock-host.d.ts", "default": "./dist/cjs/mock-host.js" }
+    }
   },
   "bin": {
     "trek-plugin-sdk": "./dist/cli/trek-plugin.js",
@@ -23,7 +29,7 @@
   },
   "files": ["dist"],
   "scripts": {
-    "build": "tsc -p tsconfig.json",
+    "build": "tsc -p tsconfig.json && tsc -p tsconfig.cjs.json && node scripts/finish-cjs.mjs",
     "typecheck": "tsc -p tsconfig.json --noEmit",
     "test": "vitest run",
     "prepublishOnly": "npm run build && npm test"