feat: Wire Gemma 4 vision into browser_tool for screenshot analysis

Default browser_vision screenshots to google/gemma-4-27b-it (Gemma 4
native multimodal) for reduced latency and unified text+vision model.

Resolution order for _get_vision_model():
1. BROWSER_VISION_MODEL env var (new, browser-specific override)
2. auxiliary.browser_vision.model in config.yaml (new config key)
3. AUXILIARY_VISION_MODEL env var (existing global vision override)
4. Default: google/gemma-4-27b-it

Backward compatibility: existing AUXILIARY_VISION_MODEL users are
unaffected — their override still flows through to browser_vision.

Also documents the new auxiliary.browser_vision config section in
cli-config.yaml.example and adds 14 unit tests covering the full
priority chain.

Fixes #816

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/tools/test_browser_vision_model.py

@@ -1,4 +1,12 @@
-"""Tests for browser_tool._get_vision_model() — Gemma 4 default (Issue #816)."""
+"""Tests for browser_tool._get_vision_model() — Gemma 4 default (Issue #816).
+
+Covers acceptance criteria from issue #816:
+- Browser screenshots use Gemma 4 by default.
+- BROWSER_VISION_MODEL env var overrides the model for browser vision only.
+- AUXILIARY_VISION_MODEL env var still works as a global override.
+- auxiliary.browser_vision.model in config.yaml overrides the default.
+- Priority: BROWSER_VISION_MODEL > config.yaml > AUXILIARY_VISION_MODEL > default.
+"""
 
 import importlib
 import sys
@@ -96,3 +104,26 @@ class TestGetVisionModelConfig:
         with patch("hermes_cli.config.load_config", return_value=cfg):
             import tools.browser_tool as bt
             assert bt._get_vision_model() == "openai/gpt-4o"
+
+    def test_config_beats_auxiliary_vision_model(self, monkeypatch):
+        """Config should override AUXILIARY_VISION_MODEL when BROWSER_VISION_MODEL unset."""
+        monkeypatch.delenv("BROWSER_VISION_MODEL", raising=False)
+        monkeypatch.setenv("AUXILIARY_VISION_MODEL", "global-override")
+        cfg = {"auxiliary": {"browser_vision": {"model": "config-model"}}}
+        with patch("hermes_cli.config.load_config", return_value=cfg):
+            import tools.browser_tool as bt
+            assert bt._get_vision_model() == "config-model"
+
+
+class TestBackwardCompatibility:
+    """AUXILIARY_VISION_MODEL must still work for users who already have it configured."""
+
+    def test_existing_auxiliary_vision_model_not_broken(self, monkeypatch):
+        """Users who set AUXILIARY_VISION_MODEL must not be broken by this change."""
+        monkeypatch.delenv("BROWSER_VISION_MODEL", raising=False)
+        monkeypatch.setenv("AUXILIARY_VISION_MODEL", "openai/gpt-4o")
+        import tools.browser_tool as bt
+        with patch("hermes_cli.config.load_config", return_value={}):
+            model = bt._get_vision_model()
+        assert model == "openai/gpt-4o"
+        assert model != "google/gemma-4-27b-it"