Add basic reference implementation for Sign Protocol

[kaiadachi]

Description

Added a basic reference implementation to demonstrate the core functionality of Sign Protocol.

Changes

• Added SignProtocolExample contract with basic schema registration
• Implemented simple attestation flow
• Added clear documentation and comments
• Focused on showing essential features for easy understanding

Why

This reference implementation will help new developers understand how to integrate with Sign Protocol by providing a clear, minimal example.

Test

 forge test      
[⠒] Compiling...
No files changed, compilation skipped

Ran 1 test for test/SignProtocolExample.t.sol:SignProtocolExample
[PASS] test_attestationFlow() (gas: 297192)
Suite result: ok. 1 passed; 0 failed; 0 skipped; finished in 13.47ms (3.51ms CPU time)

Ran 1 test suite in 169.86ms (13.47ms CPU time): 1 tests passed, 0 failed, 0 skipped (1 total tests)

[boyuanx]

@kaiadachi Thank you for this! However, could you resolve the failed check? Thanks again!

[kaiadachi]

@kaiadachi Thank you for this! However, could you resolve the failed check? Thanks again!

hi! @boyuanx
The CI environment uses bun install, but this doesn't correctly resolve the forge-std GitHub dependency in package.json. Since the remappings reference node_modules, this causes build failures.

you should either:

1. Use npm install consistently across all environments
2. Move forge-std management to Forge's lib directory via forge install

Since this project doesn't use lib/, switching from bun install to npm install would be the simplest solution.
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