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

website/docs/user-guide/features/tools.md

@@ -104,7 +104,7 @@ hermes config set terminal.singularity_image ~/python.sif
 ### Modal (Serverless Cloud)
 
 ```bash
-uv pip install "swe-rex[modal]"
+uv pip install modal
 modal setup
 hermes config set terminal.backend modal
 ```