refactor: DRY cleanup from code review

- dep_ensure.py: use get_hermes_home() instead of hand-rolled env var
- dep_ensure.py: add "chrome" to browser name list (was inconsistent with browser_tool.py)
- main.py _cmd_update_check: use detect_install_method() directly instead of redundant .git check
- main.py _cmd_update_pip: build command list directly instead of fragile split() on display string
- banner.py: rename _check_via_pypi → check_via_pypi (cross-module public API)

tests/hermes_cli/test_banner_pip_update.py

@@ -1,29 +1,29 @@
 from unittest.mock import patch
 
 
-def test_check_via_pypi_detects_update():
-    """_check_via_pypi returns 1 when PyPI has newer version."""
-    from hermes_cli.banner import _check_via_pypi
+def testcheck_via_pypi_detects_update():
+    """check_via_pypi returns 1 when PyPI has newer version."""
+    from hermes_cli.banner import check_via_pypi
     with patch("hermes_cli.banner.VERSION", "0.12.0"):
         with patch("hermes_cli.banner._fetch_pypi_latest", return_value="0.13.0"):
-            result = _check_via_pypi()
+            result = check_via_pypi()
             assert result == 1
 
 
-def test_check_via_pypi_up_to_date():
-    """_check_via_pypi returns 0 when versions match."""
-    from hermes_cli.banner import _check_via_pypi
+def testcheck_via_pypi_up_to_date():
+    """check_via_pypi returns 0 when versions match."""
+    from hermes_cli.banner import check_via_pypi
     with patch("hermes_cli.banner.VERSION", "0.13.0"):
         with patch("hermes_cli.banner._fetch_pypi_latest", return_value="0.13.0"):
-            result = _check_via_pypi()
+            result = check_via_pypi()
             assert result == 0
 
 
-def test_check_via_pypi_network_failure():
-    """_check_via_pypi returns None on network error."""
-    from hermes_cli.banner import _check_via_pypi
+def testcheck_via_pypi_network_failure():
+    """check_via_pypi returns None on network error."""
+    from hermes_cli.banner import check_via_pypi
     with patch("hermes_cli.banner._fetch_pypi_latest", return_value=None):
-        result = _check_via_pypi()
+        result = check_via_pypi()
         assert result is None