fix(backend): robustness pass — spawn leak, timeouts, workspace fail-loud, 409 order, respond guard

Audit findings 4.1-4.6.

- spawnFor: a rejected configure/start no longer leaks a registered runtime
  with a live Pi child (identity-checked teardown + rethrow); every later
  start used to hit "session runtime already active".
- ThtRunner.run: default 60s timeout on every tht child (SIGKILL backstop),
  120s for DWH-touching calls (sql preview/export, search pack); a dropped
  VPN mid-call no longer wedges the HTTP request forever.
- configArg: a NAMED workspace whose yaml is missing now throws instead of
  silently falling back to the default config (operations were silently
  targeting the wrong workspace).
- resume: the finalized/archived 409 is evaluated BEFORE the alreadyActive
  fast-path — the manifest is the truth even with a lingering runtime.
- ollamaEnsure: exit-0 with non-JSON stdout is a failed check, not ok:true.
- SessionBridge.respond: only the response matching the pending descriptor
  is forwarded to Pi; stale/duplicate submissions return 409 instead of
  being sent with the current gate's RPC id.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/test/routes-sessions.test.ts

@@ -1349,7 +1349,7 @@ test("POST resume tears down a created runtime when bridge binding fails", async
   expect(delivered).toEqual(["before", "post-failure probe"]);
 });
 
-test("POST /sessions/:id/response inoltra al bridge (no error)", async () => {
+test("POST /sessions/:id/response senza gate pendente risponde 409 (risposta stantia)", async () => {
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
     thtRunner: {
       ollamaEnsure: async () => ({ ok: true }),
@@ -1361,9 +1361,11 @@ test("POST /sessions/:id/response inoltra al bridge (no error)", async () => {
     spawnFn: () => nodeSpawn("node", [FAKE, SCRIPT]) as any,
   });
   await app.inject({ method: "POST", url: "/sessions", payload: { question: "q" } });
+  // The fake Pi never emitted a ui_request: the bridge has no pending descriptor, so a
+  // response (stale UI, double submit) must be rejected instead of forwarded to Pi.
   const res = await app.inject({ method: "POST", url: "/sessions/s1/response",
     payload: { ui_response: { id: "u1", choices: ["a"] } } });
-  expect(res.statusCode).toBe(204);
+  expect(res.statusCode).toBe(409);
 });
 
 test("POST /sessions/:id/rename calls setName", async () => {