Add offline MCP server and agent setup guide

[jakthom]

Summary

• expose a Streamable HTTP MCP endpoint at /mcp with tools for configuration, health, and delivery statistics
• reuse Beacon validation, SQLite persistence, hot reconciliation, and operational statistics for MCP calls
• add the mcp header link and a fully embedded /ui/mcp reference with setup instructions for Codex, Claude Code, Gemini CLI, and VS Code
• reserve the MCP route from sink paths and document the offline/admin-network behavior

Testing

• go test ./...
• npm test
• git diff --check