feat: Add CI workflow for smart contract build, test & lint

[Timothy2025-20]

• Add GitHub Actions workflow for contract CI
• Run cargo test, clippy, and fmt on push/PR
• Cache Cargo registry and target directory
• Build WASM artifacts
• Add coverage reporting
• Add local CI test script

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Dependency update
• CI/CD improvement

Related Issues

Closes #

Testing

• Unit tests added/updated
• Integration tests added/updated
• E2E tests added/updated (if applicable)
• Manual testing performed

Documentation

• README updated (if applicable)
• API documentation updated (if applicable)
• Code comments added for complex logic
• Migration guide added (if breaking changes)

Breaking Changes

• No breaking changes
• Breaking changes documented below:

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests passed locally with my changes
• Any dependent changes have been merged and published

Screenshots (if applicable)

Additional Context

Closes #668

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