Add CODE_OF_CONDUCT.md at repo root for inheritance

[JacobPEvans]

Problem

The .github repo has no CODE_OF_CONDUCT.md at the root level. GitHub's community health file inheritance requires this file at the root of the .github repo for it to propagate to all org repos.

Evidence

Running gh api repos/JacobPEvans/ai-workflows/community/profile shows code_of_conduct: MISSING even though this is a new repo that should inherit from .github.

Suggested Fix

Add a CODE_OF_CONDUCT.md at the root of the .github repo using the Contributor Covenant template.

Found during ai-workflows inheritance audit.