nixos: register nixos MCP server for Claude Code

Add a module that exposes the pinned mcp-nixos binary and idempotently
merges a `nixos` stdio server into the user-scope ~/.claude.json on every
switch, giving Claude Code accurate NixOS package/option search.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nixos/claude-mcp.nix

@@ -0,0 +1,58 @@
+{
+  pkgs,
+  config,
+  lib,
+  makeEnable,
+  ...
+}: let
+  # The MCP-NixOS server (https://mcp-nixos.io) — gives Claude accurate NixOS
+  # package/option/Home-Manager/flake search instead of hallucinated names.
+  # Pinned by Nix from nixpkgs, so no runtime `uvx`/`nix run` fetch is needed.
+  mcpNixosBin = "${pkgs.mcp-nixos}/bin/mcp-nixos";
+
+  # Claude Code reads MCP server *definitions* from the top-level `mcpServers`
+  # key of ~/.claude.json (the user scope). That is the only scope that applies
+  # to every project without an approval prompt while remaining additive:
+  #   - settings.json (our nix-managed dotfiles file) cannot define servers,
+  #     only filter them (enabled/disabledMcpjsonServers).
+  #   - /etc/claude-code/managed-mcp.json would take *exclusive* control and
+  #     disable every other server (per-project playwright, future `mcp add`).
+  # So we merge into the user config rather than owning a whole file.
+  serverJson = builtins.toJSON {
+    type = "stdio";
+    command = mcpNixosBin;
+  };
+in
+  makeEnable config "myModules.claudeMcpNixos" true {
+    # Also expose the pinned binary on PATH for manual `claude mcp` use / Codex.
+    environment.systemPackages = [pkgs.mcp-nixos];
+
+    # Use a module function so `lib` here is home-manager's lib (which carries
+    # `lib.hm.dag`), not the plain NixOS lib.
+    home-manager.users.imalison = {lib, ...}: {
+      # ~/.claude.json is mutable state owned by Claude Code, so it can't be
+      # managed as a whole file. Instead idempotently merge our server into it
+      # on every switch with jq, preserving all other (per-project, user-added)
+      # servers and state. This module is the declarative source of truth:
+      # `claude mcp remove nixos` is re-applied on the next switch.
+      home.activation.registerMcpNixos = lib.hm.dag.entryAfter ["writeBoundary"] ''
+        config="$HOME/.claude.json"
+        server=${lib.escapeShellArg serverJson}
+        if [ -f "$config" ]; then
+          tmp="$(mktemp)"
+          if ${pkgs.jq}/bin/jq --argjson srv "$server" \
+            '.mcpServers = ((.mcpServers // {}) + {nixos: $srv})' \
+            "$config" > "$tmp"; then
+            mv -f "$tmp" "$config"
+          else
+            rm -f "$tmp"
+            echo "claude-mcp: failed to update $config; left unchanged" >&2
+          fi
+        else
+          ${pkgs.jq}/bin/jq -n --argjson srv "$server" \
+            '{mcpServers: {nixos: $srv}}' > "$config"
+          chmod 600 "$config"
+        fi
+      '';
+    };
+  }

nixos/configuration.nix

@@ -12,6 +12,7 @@
     ./cache-server.nix
     ./cache.nix
     ./chrome-favicon-dbus.nix
+    ./claude-mcp.nix
     ./claude-remote-control.nix
     ./code.nix
     ./cua.nix