fix(cli): stop --print-effective-config from starting a container

argparse requires exactly one action, so the documented diagnostic form
`--print-effective-config --config … --start` never took the early-return
branch in main.run(): it printed the resolved config and then executed a
real do_start(), silently replacing a running container with the [default]
model. README, installation guide and manual all recommended that form.

Make it an action in the mutually exclusive group. It can no longer be
combined with --start/--check/--stop/--change/--chat (argparse error,
exit 2), and it skips the docker_available() check, so it now really is
the offline config check the docs promise. --dry-run remains a modifier
and still requires a reachable daemon.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_main.py

@@ -0,0 +1,57 @@
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "src"))
+
+from llamacppctl import actions, main as main_mod  # noqa: E402
+
+CONFIG = """
+[default]
+hf_home = /srv/models
+model_path = qwen3/default.gguf
+container_name = test_main
+"""
+
+
+def _config(tmp_path: Path) -> Path:
+    path = tmp_path / "llama.cpp.config"
+    path.write_text(CONFIG, encoding="utf-8")
+    return path
+
+
+def test_print_effective_config_has_no_side_effects(tmp_path, monkeypatch, capsys):
+    """Regression: --print-effective-config used to fall through into do_start()
+    because argparse always required an action, silently replacing a running
+    container."""
+    started = []
+    monkeypatch.setattr(actions, "do_start", lambda *a, **k: started.append(1))
+    monkeypatch.setattr(actions, "do_change", lambda *a, **k: started.append(1))
+    monkeypatch.setattr(actions, "do_stop", lambda *a, **k: started.append(1))
+
+    rc = main_mod.run(["--print-effective-config", "--config", str(_config(tmp_path))])
+
+    assert rc == 0
+    assert started == []
+    assert '"container_name": "test_main"' in capsys.readouterr().out
+
+
+def test_print_effective_config_works_without_docker(tmp_path, monkeypatch, capsys):
+    """It is a pure config check, so it must not require a reachable daemon."""
+    def boom() -> bool:
+        raise AssertionError("docker_available() must not be consulted")
+
+    monkeypatch.setattr(main_mod, "docker_available", boom)
+
+    rc = main_mod.run(["--print-effective-config", "--config", str(_config(tmp_path))])
+
+    assert rc == 0
+    assert '"hf_home": "/srv/models"' in capsys.readouterr().out
+
+
+def test_start_still_requires_docker(tmp_path, monkeypatch, capsys):
+    monkeypatch.setattr(main_mod, "docker_available", lambda: False)
+
+    rc = main_mod.run(["--start", "--config", str(_config(tmp_path))])
+
+    assert rc == 1
+    assert "docker is not available" in capsys.readouterr().err