fix(tools): stop unusable browser tools from crowding out browser_nav…

[MummIndia]

Summary

Tool retrieval returns a top-K (8 by default), and @playwright/mcp alone exposes
30 tools. Measured on three typical browsing requests, the agent was handed
browser_drop, browser_handle_dialog, browser_close, browser_console_messages
and the whole xy-mouse family — while browser_navigate, the only tool that opens a
page, was missing in two cases out of three. On "click the login button", six of the
eight slots went to mouse primitives.

Without the tool that opens a page, every other browser tool is dead weight, so the
model answers that it cannot reach the internet. That answer is correct, but it reads
as a refusal or a hallucination, which is what makes it expensive to diagnose: the
browser is installed, the MCP server reports 30 tools, and calling browser_navigate
by hand works fine.

This adds two guards in src/tool_index.py. MCP_INDEX_DENIED keeps 18 unusable
tools out of the retrieval index — pointer primitives (browser_click and
browser_hover work off the accessibility snapshot and need no coordinates),
debugging aids, session plumbing, and browser_run_code_unsafe, which runs arbitrary
JavaScript in the page. They stay connected and callable; they just no longer compete
for a slot. MCP_COMPANIONS pulls browser_navigate and browser_snapshot in
whenever any browser tool is retrieved. Matching is on the bare tool name, after the
server prefix, so neither guard depends on the browser being registered as
builtin_browser — verified with a second prefix.

Target branch

• This PR targets dev, not main.

Linked Issue

Fixes #5763

Type of Change

• Bug fix (non-breaking — fixes a confirmed issue)

Checklist

• I searched open issues and open PRs — this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above — no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up) and verified the change works end-to-end.

How to Test

1. Run the app with the built-in browser MCP server connected. Startup logs should
   show MCP server connected: Built-in: Browser (builtin_browser) - 30 tools via stdio.
2. In agent mode, send: Open https://example.com and summarise the page.
3. Before the change — check the [tool-rag] Retrieved tools for query: log line.
   mcp__builtin_browser__browser_navigate is absent, while entries such as
   browser_drop, browser_close or browser_console_messages occupy the slots.
   The agent then reports that it cannot access the internet.
4. After the change — the same log line contains browser_navigate and
   browser_snapshot, and startup reports Indexed 12 MCP tools instead of 30.
5. The agent completes the request, emitting
   Tool executed: mcp__builtin_browser__browser_navigate -> exit_code=0, then
   follows up with browser_snapshot to read the page.

Measured on three requests after the change: browser_navigate present 3/3, no
denied tool leaking through, and every slot filled with something usable —
navigate, snapshot, find, click, type, press_key, select_option.

Visual / UI changes

None. This PR only touches src/tool_index.py, which has no rendering path — no
HTML, CSS, SVG or static/js/ module is modified.