First Class Agent Support and new Toolset feature

[patvice]

Summary

This PR adds a first-class MCP agent/toolset integration so agents can declare which MCP tools to load declaratively and have them automatically wired into ask.

What changed

• Added a reusable toolset API in RubyLLM::MCP:
  • RubyLLM::MCP.toolset(:name, ...) for declaring reusable MCP tool groups.
  • RubyLLM::MCP.toolsets for introspection/access.
  • Toolset option aliases supported:
    • from_clients / clients / client_names
    • include_tools / include
    • exclude_tools / exclude
• Added new RubyLLM::MCP::Toolset class:
  • Supports filtering tools by configured MCP clients
  • Supports include/exclude filters
  • De-duplicates tools by name
  • Can materialize resolved tools via #tools / #to_a
• Added RubyLLM::MCP::Agents concern for RubyLLM::Agent classes:
  • New DSL methods: with_toolsets (with_mcp_tools alias), with_mcps
  • Agent subclasses automatically resolve and attach MCP tools in ask when configured
  • Supports inheritance of toolset/client config
  • Supports combined toolset + direct MCP client resolution with clear configuration errors for unknown names
• Updated MCP core API and behavior:
  • Added toolset option mapping + apply_toolset_options helper in lib/ruby_llm/mcp.rb
• Docs/site updates:
  • Added Agents guide link to advanced docs (docs/guides/index.md)
  • Added copy-markdown button and supporting JS/CSS/theme behavior in docs assets (docs/assets/js/copy-markdown.js, docs/_includes/head_custom.html, docs/_sass/custom/custom.scss, docs/_config.yml)

Intended usage example

# config initializer or bootstrapping code
RubyLLM::MCP.toolset(:support, clients: %i[filesystem], include_tools: %w[read_file], exclude: %w[delete_file])

# or block style
RubyLLM::MCP.toolset(:agent_messages) do |ts|
  ts.from_clients :agent_stdio
  ts.include_tools :list_messages
end

class ResearchAgent < RubyLLM::Agent
  include RubyLLM::MCP::Agents

  model "gpt-4.1"
  with_toolsets :support
  # with_mcps :agent_stdio # optional direct MCP client wiring
  # with_mcp_tools :support # alias retained for convenience
end

agent = ResearchAgent.new
agent.ask("Can you check the latest messages and summarize?")