feat: Add recent project selector (with fzf)

[daltonbr]

This PR implements an interactive launcher for recent Unity projects, as detailed in Discussion #1

Summary:

• Running the script with no arguments now opens an interactive list of recent projects, using fzf if available (with a fallback).
• The original/legacy behavior of opening a project from the current path without specifying it was preserved.
• Adds supporting files for future testing projects-v1.json and supporting documentation (also AGENTS.md for LLMs).