Add MCP server and plugin MCP discovery support

[saurabhsharma2u]

Motivation

• Introduce a Model Context Protocol (MCP) bridge so Crawlith CLI workflows can be exposed as MCP tools and prompts.
• Allow plugins to declaratively or imperatively register MCP tools/prompts during startup so third-party plugins can extend the MCP surface.
• Provide a standalone @crawlith/mcp package to run an MCP server that invokes the Crawlith CLI and aggregates plugin-provided definitions.

Description

• Extend core plugin types by adding PluginMcpTool, PluginMcpPrompt, PluginMcpDiscovery, and a mcpDiscovery field on PluginContext, and expose mcp and onMcpDiscovery on CrawlithPlugin for declarative and hook-based discovery.
• Add a new package @crawlith/mcp with an MCP server implementation in src/mcp-server.ts that registers built-in tools (ensure_crawlith_cli, crawl_site, analyze_page, probe_domain, list_sites) and built-in prompts via src/prompts.ts.
• Implement plugin discovery in the MCP server by loading repository plugins (via PluginLoader), merging declarative plugin.mcp entries and running hooks.onMcpDiscovery(ctx) with a ctx.mcpDiscovery API that calls registerTool/registerPrompt.
• Provide CLI resolution and execution utilities including resolveCliCommand, runCliCommand, and ensureCliAvailable (with optional install via npm/pnpm/yarn/bun) and add package metadata and README for the new package.

Testing

• No automated tests were run for this change.

---

Codex Task