feat: wire Gemma 4 vision into browser_tool for screenshot analysis

Default browser screenshot analysis now uses Gemma 4 27B
(google/gemma-4-27b-it) instead of deferring to the auxiliary router's
auto-detection.  Gemma 4 is natively multimodal — the same model family
already in use for text tasks — which avoids cold-start model-switching
overhead and improves context continuity.

Resolution order for _get_vision_model():
  1. BROWSER_VISION_MODEL env var (browser-specific override)
  2. auxiliary.browser_vision.model in config.yaml
  3. AUXILIARY_VISION_MODEL env var (shared/legacy override)
  4. google/gemma-4-27b-it (new default)

- Add _BROWSER_VISION_DEFAULT_MODEL constant to browser_tool.py
- Document auxiliary.browser_vision config key in cli-config.yaml.example
- Add 10 unit tests covering all resolution steps

Fixes #816

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

cli-config.yaml.example

@@ -348,7 +348,7 @@ compression:
 # Other providers pick a sensible default automatically.
 #
 # auxiliary:
-#   # Image analysis: vision_analyze tool + browser screenshots
+#   # Image analysis: vision_analyze tool
 #   vision:
 #     provider: "auto"
 #     model: ""              # e.g. "google/gemini-2.5-flash", "openai/gpt-4o"
@@ -356,6 +356,13 @@ compression:
 #     download_timeout: 30   # Image HTTP download timeout (seconds)
 #                            # Increase for slow connections or self-hosted image servers
 #
+#   # Browser screenshot analysis (browser_vision tool)
+#   # Defaults to Gemma 4 27B — natively multimodal, same model family as the main
+#   # text model, which avoids model-switching overhead and improves context continuity.
+#   # Override with any vision-capable model.  Set to "" to fall back to auto-detection.
+#   browser_vision:
+#     model: "google/gemma-4-27b-it"  # default; override e.g. "google/gemini-2.5-flash"
+#
 #   # Web page scraping / summarization + browser page text extraction
 #   web_extract:
 #     provider: "auto"