feat(config): detect pip install method and recommend correct update command

Adds detect_install_method() to identify nixos/homebrew/git/pip installs,
and recommended_update_command_for_method() to return the right upgrade command
for each method. Updates recommended_update_command() to use these for pip-installed
instances (no .git dir, not managed).

tests/hermes_cli/test_pip_install_detection.py

@@ -0,0 +1,37 @@
+from pathlib import Path
+from unittest.mock import patch
+
+
+def test_pip_install_detected_when_no_git_dir(tmp_path):
+    """When PROJECT_ROOT has no .git, detect as pip install."""
+    with patch("hermes_cli.config.get_managed_system", return_value=None):
+        from hermes_cli.config import detect_install_method
+        method = detect_install_method(project_root=tmp_path)
+        assert method == "pip"
+
+
+def test_git_install_detected_when_git_dir_exists(tmp_path):
+    """When PROJECT_ROOT has .git, detect as git install."""
+    (tmp_path / ".git").mkdir()
+    with patch("hermes_cli.config.get_managed_system", return_value=None):
+        from hermes_cli.config import detect_install_method
+        method = detect_install_method(project_root=tmp_path)
+        assert method == "git"
+
+
+def test_managed_install_takes_precedence(tmp_path):
+    """When HERMES_MANAGED is set, that takes precedence over git detection."""
+    (tmp_path / ".git").mkdir()
+    with patch("hermes_cli.config.get_managed_system", return_value="NixOS"):
+        from hermes_cli.config import detect_install_method
+        method = detect_install_method(project_root=tmp_path)
+        assert method == "nixos"
+
+
+def test_recommended_update_command_pip():
+    """Pip installs recommend pip install --upgrade."""
+    from hermes_cli.config import recommended_update_command_for_method
+    cmd = recommended_update_command_for_method("pip")
+    assert "pip install" in cmd or "uv pip install" in cmd
+    assert "--upgrade" in cmd
+    assert "hermes-agent" in cmd