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

website/docs/reference/toolsets-reference.md

@@ -52,7 +52,7 @@ Or in-session:
 
 | Toolset | Tools | Purpose |
 |---------|-------|---------|
-| `browser` | `browser_back`, `browser_click`, `browser_close`, `browser_console`, `browser_get_images`, `browser_navigate`, `browser_press`, `browser_scroll`, `browser_snapshot`, `browser_type`, `browser_vision`, `web_search` | Full browser automation. Includes `web_search` as a fallback for quick lookups. |
+| `browser` | `browser_back`, `browser_click`, `browser_console`, `browser_get_images`, `browser_navigate`, `browser_press`, `browser_scroll`, `browser_snapshot`, `browser_type`, `browser_vision`, `web_search` | Full browser automation. Includes `web_search` as a fallback for quick lookups. |
 | `clarify` | `clarify` | Ask the user a question when the agent needs clarification. |
 | `code_execution` | `execute_code` | Run Python scripts that call Hermes tools programmatically. |
 | `cronjob` | `cronjob` | Schedule and manage recurring tasks. |