Add instructions for installing/running to top-level README

[KingMob]

As the first top-level file most people see, it should contain instructions for installing and running by itself.

• The actual gt sling commands are buried in the formulas/README.md near the end of the file. It also doesn't indicate if you should run them at the terminal, or inside Claude.
• epic-delivery is mentioned, but should have install instructions, too.
• Maybe also move install instructions for Codex/Gemini to top-level README, or link to their respective sites.

[KingMob]

Also, the gt sling commands in the formulas/README.md use <crew> with angle brackets, which suggests crew is mandatory. I'd recommend switching to [crew] to indicate it's optional. (It is optional with gt-toolkit, right?)