fix(backend): tht --config is per-command, append after subcommand (not global -c)

Live end-to-end surfaced a 500 on every tht call: ThtRunner prepended
'-c <config>' before the subcommand, but tht has no global -c option
('No such option: -c'). --config/-c is a per-command option, so it must be
appended AFTER the subcommand. Extracted buildArgv() and fixed the unit test
that had codified the wrong (prepended) order.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/tht/tht-runner.ts

@@ -33,9 +33,18 @@ export class ThtRunner {
     return ["-c", this.cfg.configPath];
   }
 
+  /**
+   * Build the full argv for a `tht` invocation. `--config`/`-c` is a PER-COMMAND
+   * option in the `tht` CLI (there is NO global `-c`), so it MUST be appended
+   * AFTER the subcommand + its flags, never prepended.
+   */
+  buildArgv(args: string[], workspace?: string): string[] {
+    return [...args, ...this.configArg(workspace)];
+  }
+
   run(args: string[], workspace?: string): Promise<{ code: number; stdout: string; stderr: string }> {
     return new Promise((resolve) => {
-      const ch = spawn(this.cfg.thtBin, [...this.configArg(workspace), ...args], {
+      const ch = spawn(this.cfg.thtBin, this.buildArgv(args, workspace), {
         cwd: this.cfg.harnessDir,
       });
       let stdout = "";