mcp: add Model Context Protocol server support

Adds a new `mcp` run mode to start an MCP server over stdio.
Implements tools for navigation and JS evaluation, along with
resources for HTML and Markdown page content.

src/mcp/protocol.zig

@@ -0,0 +1,97 @@
+const std = @import("std");
+
+pub const Request = struct {
+    jsonrpc: []const u8 = "2.0",
+    id: std.json.Value,
+    method: []const u8,
+    params: ?std.json.Value = null,
+};
+
+pub const Response = struct {
+    jsonrpc: []const u8 = "2.0",
+    id: std.json.Value,
+    result: ?std.json.Value = null,
+    @"error": ?Error = null,
+};
+
+pub const Error = struct {
+    code: i64,
+    message: []const u8,
+    data: ?std.json.Value = null,
+};
+
+pub const Notification = struct {
+    jsonrpc: []const u8 = "2.0",
+    method: []const u8,
+    params: ?std.json.Value = null,
+};
+
+// Core MCP Types mapping to official specification
+pub const InitializeRequest = struct {
+    jsonrpc: []const u8 = "2.0",
+    id: std.json.Value,
+    method: []const u8 = "initialize",
+    params: InitializeParams,
+};
+
+pub const InitializeParams = struct {
+    protocolVersion: []const u8,
+    capabilities: Capabilities,
+    clientInfo: Implementation,
+};
+
+pub const Capabilities = struct {
+    experimental: ?std.json.Value = null,
+    roots: ?RootsCapability = null,
+    sampling: ?SamplingCapability = null,
+};
+
+pub const RootsCapability = struct {
+    listChanged: ?bool = null,
+};
+
+pub const SamplingCapability = struct {};
+
+pub const Implementation = struct {
+    name: []const u8,
+    version: []const u8,
+};
+
+pub const InitializeResult = struct {
+    protocolVersion: []const u8,
+    capabilities: ServerCapabilities,
+    serverInfo: Implementation,
+};
+
+pub const ServerCapabilities = struct {
+    experimental: ?std.json.Value = null,
+    logging: ?LoggingCapability = null,
+    prompts: ?PromptsCapability = null,
+    resources: ?ResourcesCapability = null,
+    tools: ?ToolsCapability = null,
+};
+
+pub const LoggingCapability = struct {};
+pub const PromptsCapability = struct {
+    listChanged: ?bool = null,
+};
+pub const ResourcesCapability = struct {
+    subscribe: ?bool = null,
+    listChanged: ?bool = null,
+};
+pub const ToolsCapability = struct {
+    listChanged: ?bool = null,
+};
+
+pub const Tool = struct {
+    name: []const u8,
+    description: ?[]const u8 = null,
+    inputSchema: std.json.Value,
+};
+
+pub const Resource = struct {
+    uri: []const u8,
+    name: []const u8,
+    description: ?[]const u8 = null,
+    mimeType: ?[]const u8 = null,
+};