ci: security scanning, with the findings read and justified

[marcos-mendez]

Calls security-scan.yml of keel-linux/.github (gitleaks, bandit,
semgrep, shellcheck on hosted runners, no secrets) on pull requests,
pushes to main and by hand. Not a required check.

The handbook carries real scripts (key generation and rotation,
organization setup, brand export). A scan of main finds nothing for
bandit or semgrep and one shellcheck warning (SC2034, an unused
variable in a provisioning script), justified in
.github/security-baseline. gitleaks flags docs/build-host.md quoting
the fingerprint of the public TurnKey signing key, ignored by
fingerprint in .github/.gitleaksignore.

[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.