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.

A scan of main finds nothing for bandit, semgrep or shellcheck, so there
is no baseline.

gitleaks flags KEEL_APT_KEY in the Makefile, the OpenPGP fingerprint of
the archive staging signing subkey: public by design, allowlisted by
value in .github/gitleaks.toml.

[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.

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