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/src/tht/tht-runner.ts

@@ -1,4 +1,6 @@
 import { spawn } from "node:child_process";
+import { existsSync } from "node:fs";
+import { join } from "node:path";
 
 export interface ThtConfig {
   thtBin: string;
@@ -19,9 +21,21 @@ export interface SessionRow {
 export class ThtRunner {
   constructor(private cfg: ThtConfig) {}
 
-  run(args: string[]): Promise<{ code: number; stdout: string; stderr: string }> {
+  /**
+   * Resolve the `-c <config>` args. If `workspace` is given AND a matching
+   * `workspaces/<workspace>.yaml` exists under harnessDir, select it; otherwise
+   * fall back to the default configPath.
+   */
+  private configArg(workspace?: string): string[] {
+    if (workspace && existsSync(join(this.cfg.harnessDir, "workspaces", `${workspace}.yaml`))) {
+      return ["-c", `workspaces/${workspace}.yaml`];
+    }
+    return ["-c", this.cfg.configPath];
+  }
+
+  run(args: string[], workspace?: string): Promise<{ code: number; stdout: string; stderr: string }> {
     return new Promise((resolve) => {
-      const ch = spawn(this.cfg.thtBin, ["-c", this.cfg.configPath, ...args], {
+      const ch = spawn(this.cfg.thtBin, [...this.configArg(workspace), ...args], {
         cwd: this.cfg.harnessDir,
       });
       let stdout = "";
@@ -32,8 +46,8 @@ export class ThtRunner {
     });
   }
 
-  private async json<T>(args: string[]): Promise<T> {
-    const { code, stdout, stderr } = await this.run(args);
+  private async json<T>(args: string[], workspace?: string): Promise<T> {
+    const { code, stdout, stderr } = await this.run(args, workspace);
     if (code !== 0) throw new Error(`tht ${args.join(" ")} exit ${code}: ${stderr.trim()}`);
     return JSON.parse(stdout) as T;
   }
@@ -44,6 +58,7 @@ export class ThtRunner {
     model?: string;
     thinking?: string;
     name?: string;
+    workspace?: string;
   }) {
     const a = ["session", "new", o.question];
     for (const [f, v] of [
@@ -55,7 +70,7 @@ export class ThtRunner {
       if (v) a.push(f, v);
     }
     a.push("--json");
-    return this.json<{ id: string }>(a);
+    return this.json<{ id: string }>(a, o.workspace);
   }
 
   sessionList() {