chore(ci): add GitHub Actions CI/CD workflows

[slow2342]

Summary

• Add CI workflow with build (x86_64 + aarch64), test, clippy, fmt, proto, and security audit jobs
• Add release workflow for tag-based binary builds
• Add CodeQL workflow for code security scanning (SAST)
• Add Scorecards workflow for supply chain security evaluation
• Update ci-conventions.md to reflect Linux-only builds and action versions

Testing

• CI workflow triggers on push/PR to main
• Build matrix runs on both x86_64 and aarch64
• Nightly build does not block merge (continue-on-error)
• CodeQL scans Rust code and uploads to Security tab
• Scorecards runs and generates OpenSSF report
• Release workflow triggers on tag push

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.