Fixes #244: Add MCP Plan Generation Example

[davideast]

Fixes #244

This PR introduces the missing mcp-plan-generation example, demonstrating how to use the Jules SDK to create a custom MCP server. The server exposes a generate_plan tool which leverages a repoless Jules session to output a detailed implementation plan (including reference code, build/test commands, and verification checks).

The structure follows the exact boundaries defined in the original issue constraints, creating a self-contained CLI tool using citty mapped strictly inside packages/core/examples/mcp-plan-generation/index.ts. All appropriate build configurations and documentation updates were also included.

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PR created automatically by Jules for task 8376256374885445621 started by @davideast

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