Add implementation plan for Angor CLI / MCP server

[dangershony]

Summary

• Adds src/cli/PLAN.md with a detailed implementation plan for a .NET 9 console app that exposes the Angor SDK as both a CLI tool and an MCP (Model Context Protocol) server
• Covers all ~45 SDK operations (wallet, projects, founder, investor, lightning, config)
• Includes a critical architectural rule: any business logic found in UI layers must be refactored into the SDK before the CLI consumes it
• Estimated ~2 days of implementation work across 8 phases

This is a clean re-creation of #790 with only the relevant commit (9f4e303).