refactor: replace swe-rex with native Modal SDK for Modal backend (#3538)

Drop the swe-rex dependency for Modal terminal backend and use the
Modal SDK directly (Sandbox.create + Sandbox.exec). This fixes:

- AsyncUsageWarning from synchronous App.lookup() in async context
- DeprecationError from unencrypted_ports / .url on unencrypted tunnels
  (deprecated 2026-03-05)

The new implementation:
- Uses modal.App.lookup.aio() for async-safe app creation
- Uses Sandbox.create.aio() with 'sleep infinity' entrypoint
- Uses Sandbox.exec.aio() for direct command execution (no HTTP server
  or tunnel needed)
- Keeps all existing features: persistent filesystem snapshots,
  configurable resources (CPU/memory/disk), sudo support, interrupt
  handling, _AsyncWorker for event loop safety

Consistent with the Docker backend precedent (PR #2804) where we
removed mini-swe-agent in favor of direct docker run.

Files changed:
- tools/environments/modal.py - core rewrite
- tools/terminal_tool.py - health check: modal instead of swerex
- hermes_cli/setup.py - install modal instead of swe-rex[modal]
- pyproject.toml - modal extra: modal>=1.0.0 instead of swe-rex[modal]
- scripts/kill_modal.sh - grep for hermes-agent instead of swe-rex
- tests/ - updated for new implementation
- environments/README.md - updated patches section
- website/docs - updated install command

tests/tools/test_terminal_requirements.py

@@ -63,7 +63,7 @@ def test_modal_backend_without_token_or_config_logs_specific_error(monkeypatch,
     monkeypatch.setenv("TERMINAL_ENV", "modal")
     monkeypatch.setenv("HOME", str(tmp_path))
     monkeypatch.setenv("USERPROFILE", str(tmp_path))
-    # Pretend swerex is installed
+    # Pretend modal is installed
     monkeypatch.setattr(terminal_tool_module.importlib.util, "find_spec", lambda _name: object())
 
     with caplog.at_level(logging.ERROR):