improve openSSF security posture

[bburli-craft]

Refs #75

improve OpenSSF scorecard security posture

• Pin all GitHub Actions to commit hashes (Token-Permissions, Pinned-Dependencies)
• Add top-level and per-job read-only permissions to CI workflow
• Replace @latest go install calls with pinned GitHub Actions
  (golangci/golangci-lint-action, golang/govulncheck-action)
• Add CodeQL SAST workflow for automated code analysis on every push/PR
• Update SECURITY.md to use GitHub private vulnerability reporting
  instead of public issues, with SLA commitments and disclosure policy
• Add Go native fuzz tests for ksp channel parsing and retry config validation

[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.

[subhramit]

LGTM sans one enhancement