fix(gateway): add Windows support for gateway process lifecycle

[Escapingbug]

The ohmo gateway process management (start, find, stop) only worked on Unix-like systems. On Windows, all three operations fail:

• start_gateway_process: uses start_new_session=True (no equivalent on Windows; also lacks stdin=DEVNULL which causes the subprocess to inherit the parent's console input)
• _pid_is_running: uses os.kill(pid, 0) which raises PermissionError for processes the caller doesn't own on Windows
• _iter_workspace_gateway_pids: shells out to ps, which doesn't exist on Windows
• stop_gateway_process: uses os.kill(pid, SIGTERM), which is not supported on Windows

Replace each with a Windows-compatible path guarded by sys.platform:

• Start: CREATE_NEW_PROCESS_GROUP | DETACHED_PROCESS, stdin=DEVNULL
• PID check: OpenProcess/GetExitCodeProcess via ctypes (STILL_ACTIVE)
• PID listing: wmic process where commandline like ... get processid
• Stop: taskkill /F /T /PID

The Unix paths remain unchanged.

Summary

• What problem does this PR solve?
  The ohmo gateway process lifecycle management (start_gateway_process, _pid_is_running, _iter_workspace_gateway_pids, stop_gateway_process) only works on Unix-like systems. On Windows, every operation fails:

  • start_gateway_process uses start_new_session=True which has no Windows equivalent, and the subprocess inherits the parent's console input (no stdin=DEVNULL)
  • _pid_is_running uses os.kill(pid, 0) which raises PermissionError on Windows for processes the caller doesn't own
  • _iter_workspace_gateway_pids shells out to ps, which doesn't exist on Windows
  • stop_gateway_process uses os.kill(pid, SIGTERM), which is not supported on Windows

• What changed?
  Added Windows-compatible paths for each operation, guarded by sys.platform == "win32":

  • Start: CREATE_NEW_PROCESS_GROUP | DETACHED_PROCESS + stdin=DEVNULL
  • PID check: OpenProcess/GetExitCodeProcess via ctypes (checks STILL_ACTIVE = 259)
  • PID listing: wmic process where commandline like ... get processid
  • Stop: taskkill /F /T /PID

  The Unix paths remain completely unchanged.

Validation

• uv run ruff check src tests scripts
• uv run pytest -q (pre-existing failures on Windows unrelated to this change; ohmo/ directory has no test suite)
• cd frontend/terminal && npx tsc --noEmit (frontend not touched)

Notes

• Related issue: N/A
• Follow-up work: N/A

[tjb-tech]

I would not merge this as-is. The main blocker is the new Windows PID discovery path depending on wmic:

• Microsoft documents wmic as deprecated, and their current Windows client docs say it will be removed in a future Windows release. That makes it a poor new hard dependency for gateway lifecycle management.
• Even before removal, wmic process where commandline like ... is brittle for paths with quoting / escaping differences, especially around --workspace values on Windows.

The Unix-side intent here makes sense, but I think the Windows implementation should switch to a non-wmic process query path before merge. For example: PowerShell/CIM (Get-CimInstance Win32_Process) or another API-level enumeration path, plus tests around PID discovery / stop behavior.

[Escapingbug]

BTW, what is our attitude towards third-party dependencies? Maybe using psutil is a better option considering cross-platform compatibility?