feat(#2305): hive-screen-mcp — screenshot + input MCP for GUI agents

New crate hive-screen-mcp: a stdio MCP bridge activated automatically
when an agent has hyperhive.gui.enable = true. Provides five tools:

- screenshot   — grim → saves PNG, returns path for Read tool
- type_text    — wtype → Unicode text input (no daemon)
- key_press    — ydotool key → combos like ctrl+c, super+l
- mouse_move   — ydotool mousemove --absolute
- mouse_click  — ydotool click, optionally with prior move

New nix/agent-modules/screen.nix: wires the MCP bridge into
extraMcpServers.screen; adds grim + wtype to systemPackages. Adds
hyperhive.gui.screenInput option (default false) which enables the
ydotoold daemon + ydotool for mouse/keyboard injection via /dev/uinput.

screenshot and type_text work without screenInput. key_press,
mouse_move, and mouse_click return a ydotool error until ydotoold is
running and /dev/uinput is accessible in the container.

nix/agent-modules/default.nix

@@ -33,6 +33,7 @@
     ./network.nix
     ./packages.nix
     ./user.nix
+    ./screen.nix
     ./weston-vnc.nix
     (lib.mkRemovedOptionModule [ "hyperhive" "web" "useUnixSocket" ] ''
       Unix socket mode is always enabled for all agents. Remove the