fix(backend): make data root config authoritative

backend/src/pi/list-models.ts

@@ -35,6 +35,7 @@ export function createPiModelLister(cfg: AppConfig, opts: Opts = {}): () => Prom
     if (cache && now() - cache.at < ttlMs) return cache.models;
 
     const env: NodeJS.ProcessEnv = { ...process.env };
+    delete env.THT_DATA_ROOT;
     if (cfg.dataRoot !== undefined) env.THT_DATA_ROOT = cfg.dataRoot;
     const child = spawnFn(cfg.piBin, ["--mode", "rpc"], { cwd: cfg.harnessDir, env });
     child.stderr.resume();

backend/src/pi/pi-process-manager.ts

@@ -35,6 +35,7 @@ export class PiProcessManager {
       THT_SESSION: sessionId,
       THT_AUTHOR: author,
     };
+    delete env.THT_DATA_ROOT;
     if (this.cfg.dataRoot !== undefined) env.THT_DATA_ROOT = this.cfg.dataRoot;
     // pi 0.73 removed `--approve`: rpc mode is headless and its argv is intentionally minimal.
     const child = spawnFn(this.cfg.piBin, ["--mode", "rpc"], {