Add community health files (SECURITY, CONTRIBUTING, CODE_OF_CONDUCT)

[Blake-Daniel-Inv]

Adds the three community health files:

• SECURITY.md — private vulnerability reporting via GitHub, supported-versions table, and a scope/threat-model note covering envelope-encrypted credential storage.
• CONTRIBUTING.md — dev setup, the lint/test/build checks CI enforces, PR guidelines, and a 'never commit secrets' note.
• CODE_OF_CONDUCT.md — Contributor Covenant 2.1, enforcement contact set to the maintainer's GitHub profile.

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