chore: add CI for solhint, format, and test

## Description

Set up CI pipeline to run the following checks automatically:

- **solhint** - Solidity linting
- **format** - Code formatting check (e.g. prettier)
- **test** - `npx hardhat test`

## Notes

- `contracts/verifiers/PlonkVerifier.sol` is auto-generated by gnark (`Code generated by gnark DO NOT EDIT`) and must be excluded from format/lint targets.