feat(backend): support container runtime paths

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

@@ -1,5 +1,4 @@
 import { spawn as nodeSpawn, type ChildProcessWithoutNullStreams } from "node:child_process";
-import { join } from "node:path";
 import type { AppConfig } from "../config.js";
 import { RpcClient } from "../rpc/rpc-client.js";
 import { SessionBridge } from "../bridge/session-bridge.js";
@@ -11,8 +10,12 @@ export interface SessionRuntime {
   child: ChildProcessWithoutNullStreams;
 }
 
-/** Injected test double signature: produce a child process, no args needed. */
-type SpawnFn = () => ChildProcessWithoutNullStreams;
+/** Injectable child-process boundary; callbacks may ignore arguments in simpler tests. */
+type SpawnFn = (
+  command: string,
+  args: string[],
+  options: { cwd: string; env: NodeJS.ProcessEnv },
+) => ChildProcessWithoutNullStreams;
 
 export class PiProcessManager {
   private runtimes = new Map<string, SessionRuntime>();
@@ -20,30 +23,29 @@ export class PiProcessManager {
 
   constructor(private cfg: AppConfig, opts?: { spawnFn?: SpawnFn }) {
     if (opts?.spawnFn) {
-      this.spawnFn = () => opts.spawnFn!();
+      this.spawnFn = (sessionId: string, author: string) => this.spawnPi(opts.spawnFn!, sessionId, author);
     } else {
-      this.spawnFn = (sessionId: string, author: string) => {
-        const harnessVenvBin = join(cfg.harnessDir, ".venv", "bin");
-        const env: NodeJS.ProcessEnv = {
-          ...process.env,
-          THT_SESSION: sessionId,
-          THT_AUTHOR: author,
-          PATH: `${harnessVenvBin}:${process.env.PATH ?? ""}`,
-        };
-        // pi 0.73 (the @mariozechner rebrand) removed the `--approve` flag: rpc mode is
-        // headless and runs tools without an approval gate, so passing it makes pi exit
-        // with "Unknown option: --approve". Args are intentionally just `--mode rpc`.
-        const child = nodeSpawn(cfg.piBin, ["--mode", "rpc"], {
-          cwd: cfg.harnessDir,
-          env,
-        });
-        // Drain stderr so the child's stderr buffer never blocks the process.
-        child.stderr.resume();
-        return child;
-      };
+      this.spawnFn = (sessionId: string, author: string) => this.spawnPi(nodeSpawn, sessionId, author);
     }
   }
 
+  private spawnPi(spawnFn: SpawnFn, sessionId: string, author: string): ChildProcessWithoutNullStreams {
+    const env: NodeJS.ProcessEnv = {
+      ...process.env,
+      THT_SESSION: sessionId,
+      THT_AUTHOR: author,
+    };
+    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"], {
+      cwd: this.cfg.harnessDir,
+      env,
+    });
+    // Drain stderr so the child's stderr buffer never blocks the process.
+    child.stderr.resume?.();
+    return child;
+  }
+
   count(): number { return this.runtimes.size; }
 
   get(id: string): SessionRuntime | undefined { return this.runtimes.get(id); }