Fix NPX MCP server crash (skip if not installed, alternative shape to #242 / #252) (#253)

* Fix NPX MCP server crash by checking install state instead of timing out

When @playwright/mcp (or any future npx-based built-in server) isn't
already cached, npx tries to download and install it on first invoke.
That can take minutes or hang on a fresh install missing Playwright
system deps. The previous code bounded that wait with
asyncio.wait_for(mcp_manager.connect_server(...), timeout=30), but the
cancellation that wait_for fires on timeout propagates into
mcp.client.stdio.stdio_client's internal anyio task group, which
raises:

    RuntimeError: Attempted to exit cancel scope in a different task
    than it was entered in

The error fires in a sibling background task (Task exception was never
retrieved) so the surrounding try/except BaseException doesn't catch
it, and the orphaned cancel scope cascades cancellations into other
tasks in the same event loop. Running requests start failing and the
process needs a restart.

Fix: detect whether the package is already cached before invoking
connect_server, instead of trying to bound the connect with a timeout.
A new _is_npx_package_cached helper runs:

    npx --no-install <pkg> --version

The --no-install flag makes npx fail fast on a cache miss instead of
downloading, so the probe returns in <500ms either way. If the package
isn't cached, we log a warning with the exact command the user can run
to install it, and skip the server. If it is cached, we call
connect_server normally with no wait_for wrapper, so there's no
cancellation that could enter stdio_client's task group.

This removes the entire bug class instead of papering over it. No
asyncio.wait_for around stdio_client, no shielded-task leak, no
shutdown-time RuntimeError. Verified against current versions
(mcp library on Python 3.14, anyio 4.13.0) with the existing
@playwright/mcp@latest cached, and with a deliberately uncached
package spec to exercise the skip path.

* Make first-run setup explicit when NPX MCP package isn't cached

Per @pewdiepie-archdaemon review on #253:

- src/builtin_mcp.py: expand the skip-server warning into a multi-line
  block with Reason/Impact/Fix/Notes lines, so the message stands out
  in startup logs and clearly tells the user what to run.

- README.md: add 'Built-in MCP servers (optional setup)' subsection
  under Configuration, with the install command and a brief note that
  it's optional and skipped if not cached.

README.md

@@ -219,6 +219,18 @@ Docker Compose includes these by default. The bundled service ports bind to `127
 ### Optional external services
   - **Ollama** → local LLM server -- [ollama.ai](https://ollama.ai)
 
+### Built-in MCP servers (optional setup)
+
+Odysseus auto-registers a few built-in MCP servers at startup. The npx-based ones (currently the browser server, `@playwright/mcp`) only start when their npm package is already in the local npx cache. If a package isn't cached, that server is skipped with a startup log message explaining what to do, so a fresh install does not block on a multi-minute npm download or hang if Playwright system deps are missing.
+
+To enable the browser MCP (page navigation, screenshots, vision), run once:
+
+```bash
+npx -y @playwright/mcp@latest --version
+```
+
+That installs `@playwright/mcp` plus Playwright (~300MB total). Restart Odysseus and the server will register at startup.
+
 ### Ollama with Docker
 If Odysseus is running in Docker and Ollama is running on the host, add the endpoint in Settings as: