feat: add git-explorer MCP server for read-only repository access

Implements a new MCP server that provides read-only access to git
repositories using go-git. Designed for deployment verification by
comparing deployed flake revisions against source repositories.

9 tools: resolve_ref, get_log, get_commit_info, get_diff_files,
get_file_at_commit, is_ancestor, commits_between, list_branches,
search_commits.

Includes CLI commands, NixOS module, and comprehensive tests.

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

internal/mcp/server.go

@@ -118,6 +118,27 @@ This ensures option documentation matches the home-manager version the project a
 	}
 }
 
+// DefaultGitExplorerConfig returns the default configuration for the git-explorer server.
+func DefaultGitExplorerConfig() ServerConfig {
+	return ServerConfig{
+		Name:    "git-explorer",
+		Version: "0.1.0",
+		Mode:    ModeCustom,
+		Instructions: `Git Explorer MCP Server - Read-only access to git repository information.
+
+Tools for exploring git repositories:
+- Resolve refs (branches, tags, commits) to commit hashes
+- View commit logs with filtering by author, path, or range
+- Get full commit details including file change statistics
+- Compare commits to see which files changed
+- Read file contents at any commit
+- Check ancestry relationships between commits
+- Search commit messages
+
+All operations are read-only and will never modify the repository.`,
+	}
+}
+
 // Server is an MCP server that handles JSON-RPC requests.
 type Server struct {
 	store       database.Store