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/mcp.nix

@@ -2,7 +2,7 @@
 #
 # Creates a dedicated 'mcp' system user with rootless podman support
 # and a systemd service for the agent daemon.
-{ pkgs, ... }:
+{ pkgs, inputs, ... }:
 
 let
   mcpUid = 995;
@@ -29,7 +29,7 @@ in
 
     serviceConfig = {
       Type = "simple";
-      ExecStart = "/usr/local/bin/mcp-agent server --config /srv/mcp/mcp-agent.toml";
+      ExecStart = "${inputs.mcp.packages.x86_64-linux.mcp-agent}/bin/mcp-agent server --config /srv/mcp/mcp-agent.toml";
       Restart = "on-failure";
       RestartSec = 5;
 

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
   ];
 }

flake.nix

@@ -43,6 +43,9 @@
     mcdeploy.url = "git+https://git@git.wntrmute.dev/kyle/mcdeploy";
     mcdeploy.inputs.nixpkgs.follows = "nixpkgs";
 
+    mcp.url = "git+ssh://git@git.wntrmute.dev/mc/mcp.git";
+    mcp.inputs.nixpkgs.follows = "nixpkgs";
+
     # exo.url = "git+https://git.wntrmute.dev/kyle/exo";
     # exo.inputs.nixpkgs.follows = "nixpkgs";