agent: add MCP server mode with task tool

src/Config.zig

@@ -189,6 +189,7 @@ const Commands = cli.Builder(.{
             .{ .name = "interactive", .type = bool },
             .{ .name = "task", .type = ?[]const u8 },
             .{ .name = "task_attachments", .type = []const u8, .multiple = true },
+            .{ .name = "mcp", .type = bool },
         },
         .shared_options = CommonOptions,
     },
@@ -763,6 +764,12 @@ pub fn printUsageAndExit(self: *const Config, success: bool) void {
         \\                a positional script is present, any new commands
         \\                entered in the REPL are appended to that file.
         \\
+        \\--mcp           Run as an MCP server over stdio that exposes a single
+        \\                `task` tool. Each call delegates a high-level task to
+        \\                the agent and returns only the final answer to the
+        \\                MCP client. Requires --provider; cannot be combined
+        \\                with --task, -i, or a script file.
+        \\
         \\The API key is read from the environment:
         \\ANTHROPIC_API_KEY, OPENAI_API_KEY, or GOOGLE_API_KEY.
         \\Ollama does not require an API key.