fix: Modal sandbox eval infra (9 fixes for TBLite baseline)

Fixes discovered while running TBLite baseline evaluation:

1. ephemeral_disk param not supported in modal 1.3.5 - check before passing
2. Modal legacy image builder requires working pip - add ensurepip fix via
   setup_dockerfile_commands to handle task images with broken pip
3. Host cwd leaked into Modal sandbox - add /home/ to host prefix check
4. Tilde ~ not expanded by subprocess.run(cwd=) in sandboxes - use /root
5. install_pipx must stay True for swerex-remote to be available

Dependencies also needed (not in this commit):
- git submodule update --init mini-swe-agent
- uv pip install swe-rex boto3

tools/environments/modal.py

@@ -50,7 +50,7 @@ class ModalEnvironment(BaseEnvironment):
     def __init__(
         self,
         image: str,
-        cwd: str = "~",
+        cwd: str = "/root",
         timeout: int = 60,
         modal_sandbox_kwargs: Optional[Dict[str, Any]] = None,
         persistent_filesystem: bool = True,
@@ -95,6 +95,7 @@ class ModalEnvironment(BaseEnvironment):
             startup_timeout=180.0,
             runtime_timeout=3600.0,
             modal_sandbox_kwargs=sandbox_kwargs,
+            install_pipx=True,  # Required: installs pipx + swe-rex runtime (swerex-remote)
         )
 
     def execute(self, command: str, cwd: str = "", *,