fix(backend): forward workspace to tht config selection (MVP backend-side)

POST /sessions no longer silently drops `workspace`. ThtRunner.run/json
take an optional workspace; configArg() selects workspaces/<ws>.yaml when
it exists under harnessDir, else falls back to default configPath.
sessionNew threads workspace through. Route forwards b.workspace with an
MVP note (gate/Pi side still single-workspace via symlinked config/tht.yaml).

19/19 tests pass, tsc clean.

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

backend/test/routes-sessions.test.ts

@@ -7,13 +7,19 @@ import { loadConfig } from "../src/config.js";
 const FAKE = path.resolve("../harness/tests/fake_pi/fake_pi_rpc.mjs");
 const SCRIPT = path.resolve("../harness/tests/fake_pi/scripts/f1_disambiguation.json");
 
-test("POST /sessions crea e avvia, GET /sessions lista", async () => {
+test("POST /sessions crea e avvia, GET /sessions lista; inoltra workspace a sessionNew", async () => {
+  let sessionNewArg: any;
   const app = buildApp(loadConfig({ THT_HARNESS_DIR: "../harness" }), {
-    thtRunner: { sessionNew: async () => ({ id: "s1" }), sessionList: async () => [{ id: "s1" }] } as any,
+    thtRunner: {
+      sessionNew: async (o: any) => { sessionNewArg = o; return { id: "s1" }; },
+      sessionList: async () => [{ id: "s1" }],
+    } as any,
     spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any,
   });
   const created = await app.inject({ method: "POST", url: "/sessions", payload: { workspace: "w", question: "q" } });
   expect(created.json()).toEqual({ id: "s1" });
+  expect(sessionNewArg.workspace).toBe("w");
+  expect(sessionNewArg.question).toBe("q");
   const list = await app.inject({ method: "GET", url: "/sessions" });
   expect(list.json()).toEqual([{ id: "s1" }]);
 });