refactor: remove browser_close tool — auto-cleanup handles it

The browser_close tool was called in only 9% of browser sessions (13/144
navigations across 66 sessions), always redundantly — cleanup_browser()
already runs via _cleanup_task_resources() at conversation end, and the
background inactivity reaper catches anything else.

Removing it saves one tool schema slot in every browser-enabled API call.

Also fixes a latent bug: cleanup_browser() now handles Camofox sessions
too (previously only Browserbase). Camofox sessions were never auto-cleaned
per-task because they live in a separate dict from _active_sessions.

Files changed (13):
- tools/browser_tool.py: remove function, schema, registry entry; add
  camofox cleanup to cleanup_browser()
- toolsets.py, model_tools.py, prompt_builder.py, display.py,
  acp_adapter/tools.py: remove browser_close from all tool lists
- tests/: remove browser_close test, update toolset assertion
- docs/skills: remove all browser_close references

cli-config.yaml.example

@@ -539,7 +539,7 @@ platform_toolsets:
 #   terminal     - terminal, process
 #   file         - read_file, write_file, patch, search
 #   browser      - browser_navigate, browser_snapshot, browser_click, browser_type,
-#                  browser_scroll, browser_back, browser_press, browser_close,
+#                  browser_scroll, browser_back, browser_press,
 #                  browser_get_images, browser_vision  (requires BROWSERBASE_API_KEY)
 #   vision       - vision_analyze  (requires OPENROUTER_API_KEY)
 #   image_gen    - image_generate  (requires FAL_KEY)