fix(backend): harden principal child isolation

backend/src/pi/pi-process-manager.ts

@@ -5,7 +5,7 @@ import { SessionBridge } from "../bridge/session-bridge.js";
 import type { ThtRunner } from "../tht/tht-runner.js";
 import { buildPiChildEnv, canonicalPiProvider } from "./provider-credentials.js";
 import { secretValue } from "../config/secret-bundle.js";
-import { principalEnvironment, type PrincipalContext } from "../auth/principal.js";
+import { clearPrincipalEnvironment, principalEnvironment, type PrincipalContext } from "../auth/principal.js";
 
 export interface SessionRuntime {
   rpc: RpcClient;
@@ -55,6 +55,7 @@ export class PiProcessManager {
       credentialFile: this.cfg.modelApiKeyFile,
       additions: { THT_SESSION: sessionId, THT_AUTHOR: author },
     });
+    clearPrincipalEnvironment(env);
     if (principal) Object.assign(env, principalEnvironment(principal));
     // The Thoth gate executes the deterministic `tht` CLI as a Pi tool. Give only
     // this managed session process the adapter values already loaded by the core