docs: add nixos-options MCP configuration

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -63,6 +63,23 @@ Examples:
 - `template2: add proxmox image configuration`
 - `terraform: add VM deployment configuration`
 
+### NixOS Options Lookup
+
+An MCP server is available for searching and looking up NixOS option documentation. This is useful when configuring new services or options for hosts.
+
+**Session Setup:** At the start of each session, index the nixpkgs revision from `flake.lock` to ensure option documentation matches the project's nixpkgs version:
+
+1. Read `flake.lock` and find the `nixpkgs` node's `rev` field
+2. Call `index_revision` with that git hash
+
+**Usage Examples:**
+
+- `search_options` - Search for options by name or description (e.g., query "nginx" or "postgresql")
+- `get_option` - Get full details for a specific option (e.g., `services.loki.configuration`)
+- `get_file` - Fetch the source file from nixpkgs that declares an option
+
+This ensures documentation matches the exact nixpkgs version (currently NixOS 25.11) used by this flake.
+
 ## Architecture
 
 ### Directory Structure