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.

AGENTS.md

@@ -23,7 +23,7 @@ extensions/
   notify.ts               # Native terminal notification when agent finishes
   ext-toggle.ts           # /ext slash command — list & toggle extensions at runtime
   todo.ts                 # `todo` tool for the agent + /todos for the user (copy of upstream example)
-  mcp-loader.ts           # Generic MCP server loader — reads `mcp` block from settings.json
+  mcp-loader.ts           # Generic MCP server loader + /mcp slash command
 install.sh                # Idempotent installer — symlinks extensions/ into ~/.pi/agent/extensions/
 package.json              # pi package manifest — enables `pi install /path` as an alternative
 README.md                 # User-facing docs.