chore: agent skill for the BSI certification

[edulix]

Adds a bsi-certification agent skill: where the BSI evidence lives (beyond#615), which step files are evidence (VDP, security.txt, TOE production definitions) and the rules for changing them.

• The skill lives in .agents/skills/ (read by Codex); .claude/skills is a symlink to it for Claude Code.
• reuse lint passes for the new file. The branch already has 11 files without REUSE information (packages/third_party/celery-codegen, three READMEs and two tests); this PR does not change them.

AGENTS.md itself comes from main with #3879.

Targets the BSI certification branch feat/meta-10548/main (PR #2335), not main.

Related: https://github.com/sequentech/meta/issues/10548

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[edulix]

Consolidated into #2335.