test(grok): mypy-clean the reaper watchdog + interactive spawn tests

The CI mypy scope (roboco/ tests/) flagged test-only typing issues my per-file
runs missed: direct method assignment (orch._remove_container = AsyncMock())
trips [method-assign], and a module-level dict[str,str] is invariant against
the dict[str, str|None] the run-spec expects.

- Use monkeypatch.setattr for _remove_container in the watchdog tests.
- Annotate the shared _HOSTS as dict[str, str | None].

Production code unchanged; mypy roboco/ tests/ is green.

tests/unit/runtime/test_interactive_grok_spawn.py

@@ -15,7 +15,7 @@ from roboco.runtime.orchestrator import (
     _SecretaryRunSpec,
 )
 
-_HOSTS = {
+_HOSTS: dict[str, str | None] = {
     "claude": "/h/.claude",
     "prompt": "/h/p.md",
     "workspaces": "/h/ws",