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/tht-runner.test.ts

@@ -1,6 +1,23 @@
-import { test, expect } from "vitest";
+import { test, expect, vi } from "vitest";
+import { EventEmitter } from "node:events";
 import { ThtRunner } from "../src/tht/tht-runner.js";
 
+// Spy on child_process.spawn so we can capture the resolved argv (incl. -c config)
+// that ThtRunner.run() builds, without launching a real process.
+vi.mock("node:child_process", () => ({
+  spawn: vi.fn(() => {
+    const ch: any = new EventEmitter();
+    ch.stdout = new EventEmitter();
+    ch.stderr = new EventEmitter();
+    queueMicrotask(() => {
+      ch.stdout.emit("data", Buffer.from('{"id":"x"}'));
+      ch.emit("close", 0);
+    });
+    return ch;
+  }),
+}));
+import { spawn } from "node:child_process";
+
 test("sessionNew parses id from JSON", async () => {
   const r = new ThtRunner({ thtBin: "tht", harnessDir: "/h", configPath: "config/tht.yaml" });
   r.run = async () => ({ code: 0, stdout: '{"id":"2026-06-27-100000-x"}', stderr: "" });
@@ -12,3 +29,13 @@ test("run with exit != 0 propagates error with stderr", async () => {
   r.run = async () => ({ code: 1, stdout: "", stderr: "ERRORE: boom" });
   await expect(r.sessionList()).rejects.toThrow(/boom/);
 });
+
+test("sessionNew with missing workspace file falls back to default configPath argv", async () => {
+  // harnessDir "/nope" has no workspaces/foo.yaml -> configArg falls back to default.
+  (spawn as any).mockClear();
+  const r = new ThtRunner({ thtBin: "tht", harnessDir: "/nope", configPath: "config/tht.yaml" });
+  await r.sessionNew({ question: "q", workspace: "foo" });
+  const [bin, argv] = (spawn as any).mock.calls[0];
+  expect(bin).toBe("tht");
+  expect(argv).toEqual(["-c", "config/tht.yaml", "session", "new", "q", "--json"]);
+});