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>

routers/position.js

@@ -0,0 +1,14 @@
+// Reads agent position from ego.json (optional dependency on PaddedCell/ego-mgr)
+import { readFileSync } from "node:fs";
+import { homedir } from "node:os";
+
+const EGO_PATH = `${homedir()}/.openclaw/ego.json`;
+
+export function resolve(ctx) {
+  try {
+    const ego = JSON.parse(readFileSync(EGO_PATH, "utf8"));
+    return ego["agent-scope"]?.[ctx.agentId]?.position || "";
+  } catch {
+    return "";
+  }
+}