feat: PrismFacet core implementation

- Plugin scaffold: manifest, tsconfig, package.json
- Router loader with dynamic import + cache busting for hot reload
- Rule store: CRUD on rules.json
- Prompt injector: resolve routers → match rules → read files → inject
- MCP tool: prompt_rules (add/remove/list/test/reload-routers/list-routers)
- Built-in routers: agent-id (zero-dep), role, position (ego.json)
- before_prompt_build hook for system prompt injection

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

package.json

@@ -0,0 +1,14 @@
+{
+  "name": "prism-facet",
+  "version": "0.1.0",
+  "type": "module",
+  "main": "dist/index.js",
+  "scripts": {
+    "build": "tsc",
+    "watch": "tsc --watch"
+  },
+  "devDependencies": {
+    "@types/node": "^25.6.0",
+    "typescript": "^5.5.0"
+  }
+}