feat(backend): resume + venv PATH + end-to-end F1 smoke (fake-pi-rpc)

- Add PiProcessManager.resume(sessionId, tht): reads provider/model/thinking
  from tht.sessionShow() and calls spawnFor with those values
- Add POST /sessions/:id/resume route: calls mgr.resume then re-wires
  bridge.onClientEvent → hub.publish
- Confirm venv PATH already present in real spawn (no change needed)
- Add e2e test: POST /sessions → SSE receives ui_request via pendingWidget
  re-emit → POST /sessions/s1/response → 204 (all over real HTTP against
  fake-pi-rpc)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -3,6 +3,7 @@ import { join } from "node:path";
 import type { AppConfig } from "../config.js";
 import { RpcClient } from "../rpc/rpc-client.js";
 import { SessionBridge } from "../bridge/session-bridge.js";
+import type { ThtRunner } from "../tht/tht-runner.js";
 
 export interface SessionRuntime {
   rpc: RpcClient;
@@ -74,6 +75,15 @@ export class PiProcessManager {
     return rt;
   }
 
+  async resume(sessionId: string, tht: ThtRunner): Promise<SessionRuntime> {
+    const manifest = await tht.sessionShow(sessionId) as { provider?: string; model?: string; thinking?: string } | null;
+    return this.spawnFor(sessionId, {
+      provider: manifest?.provider,
+      model: manifest?.model,
+      thinking: manifest?.thinking,
+    });
+  }
+
   teardown(id: string): void {
     const rt = this.runtimes.get(id);
     if (rt) {