docs: QUICKSTART.md — install to first UserOp in under 10 minutes (PR ready)

[enochTe8]

Hi team,

I'm Enoch, an ERC-4337 technical writer.

There is currently no getting-started guide on this repo. The gap
between cloning and sending a first UserOp is completely undocumented.

I've already written:

• Full QUICKSTART.md covering install, init, first transaction
• Counterfactual address derivation
• Optional paymaster config for gasless UX
• Common error table with 8 entries

Everything is PR-ready: https://github.com/enochTe8

Happy to open the PR directly.