chore: add repobuddy/agent-security skill

[unional]

Install repobuddy/agent-security skill for automated security PR fixing.

[changeset-bot]

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Latest commit: cc6fce9

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[unional]

Left open untouched — this is your own skill-vendoring work, unrelated to the modernization pass, and it is your call whether to land it.

For context on why it shows as conflicted: main has moved a fair amount today (#628, #630, #631, #625, #633, and the 8.0.0-beta.9 version bump), and skills-lock.json is the kind of file that conflicts on almost any of that. The .agents/skills/fix-security-pr/SKILL.md half should rebase without trouble.