[F005,F006] grok auth: directory mount + atomic-write fallback

F005: the single-file bind mount of auth.json pinned the inode, so the
orchestrator's atomic refresh (tmp+rename within ~/.grok) never reached a
running grok container — a long-lived container hung at the login prompt
when the original ~6h token expired. Mount the host ~/.grok DIRECTORY (ro)
at /home/agent/.grok-auth-ro; the entrypoint symlinks ~/.grok/auth.json at
that RO mount so grok + the --check backstop read the live credential (the
directory mount sees the host-side rename) while grok's writable state
(config.toml, sessions/) stays in the image's ~/.grok.

F006: a rotated refresh_token is single-use — xAI invalidates the old one
the instant it issues the new one. If the atomic write failed after the
rotation, the file kept the now-dead old refresh_token and the credential
was permanently lost on the next refresh. _atomic_write now falls back to a
direct write when tmp+replace fails, so the rotated token always lands on
disk (losing the write is catastrophic; losing atomicity is not).

TDD: RED tests watched fail, then GREEN. ruff+mypy clean; 32 grok tests
green, no regressions.

tests/unit/runtime/test_interactive_grok_spawn.py

@@ -82,7 +82,8 @@ def test_intake_grok_mounts_subscription_auth_when_present(
     cmd = AgentOrchestrator._build_intake_run_cmd(
         _intake_spec("grok", base_url="https://api.x.ai/v1", token="xai-key")
     )
-    assert f"{grok_dir / 'auth.json'}:/home/agent/.grok/auth.json:ro" in cmd
+    # F005: directory mount (ro), not the single-file inode-pinning mount.
+    assert f"{grok_dir}:/home/agent/.grok-auth-ro:ro" in cmd
 
 
 def test_intake_anthropic_keeps_anthropic_env() -> None: