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/routes/sessions.ts

@@ -31,6 +31,12 @@ export function sessionRoutes(app: FastifyInstance, d: { mgr: PiProcessManager;
     rt.bridge.steer((req.body as any).text);
     return reply.code(204).send();
   });
+  app.post("/sessions/:id/resume", async (req, reply) => {
+    const id = (req.params as any).id;
+    const rt = await d.mgr.resume(id, d.tht);
+    rt.bridge.onClientEvent((e) => d.hub.publish(id, e.type, e));
+    return reply.code(200).send({ id });
+  });
   app.post("/sessions/:id/close", async (req) => {
     d.mgr.teardown((req.params as any).id);
     return { closed: true };