Add MCP to Nix packages and wire agent to Nix-managed binary

- Add mcp flake input (git+ssh://git@git.wntrmute.dev/mc/mcp.git)
- Add mcp CLI to mcpkg.nix system packages (installed on all machines)
- Update mcp.nix to use Nix-managed mcp-agent binary path instead of
  hardcoded /usr/local/bin/mcp-agent

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

configs/mcpkg.nix

@@ -6,5 +6,6 @@
     inputs.mcr.packages.x86_64-linux.default
     inputs.mc-proxy.packages.x86_64-linux.default
     inputs.mcdeploy.packages.x86_64-linux.default
+    inputs.mcp.packages.x86_64-linux.default
   ];
 }