mcp-loader: add /mcp slash command for runtime status + toggle

Mirrors /ext UX (space=stage, enter=apply+reload, esc=cancel) but for
MCP servers in the settings.json `mcp` block. Tracks per-server runtime
state captured at extension load time so users can see at a glance
which servers are running / failed / disabled / remote-skipped /
invalid, with tool counts for the running ones.

Toggling writes back to settings.json — disabling sets enabled:false,
re-enabling removes the explicit key (default is true) to keep the
file tidy. Then ctx.reload() picks up the change.

Closes the visibility gap surfaced by 'searxng_search isn't in /ext':
MCP-provided tools are runtime-spawned, not file-based extensions, so
they need their own list view. /mcp fills that hole.

README.md

@@ -250,6 +250,16 @@ Generic MCP server loader. Reads an `mcp` block from `~/.pi/agent/settings.json`
 
 **Debug:** set `PI_MCP_LOADER_DEBUG=1` in the environment to surface per-server stderr and connection logs.
 
+**Slash command:** `/mcp` opens a multi-toggle overlay listing every server in the `mcp` block with its runtime status:
+
+- `running · N tools` — connected, tools registered
+- `failed: <message>` — start handshake threw
+- `disabled in settings` — `enabled: false`
+- `remote (skipped — v1 stdio only)` — type `remote`, awaiting v2
+- `invalid: <message>` — malformed config (read-only row)
+
+UX matches `/ext`: **space** stages a toggle, **enter** writes back to `settings.json` and reloads pi, **esc** cancels. Toggling re-enables a previously-disabled server by removing the explicit `enabled` key (the default is `true`).
+
 Each extension is a TypeScript module loaded by [jiti](https://github.com/unjs/jiti) — no compilation step. See the [pi extensions docs](https://github.com/mariozechner/pi-coding-agent/blob/main/docs/extensions.md) and the [built-in examples](https://github.com/mariozechner/pi-coding-agent/tree/main/examples/extensions) for the API surface.
 
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