fix: docs: add CODE_OF_CONDUCT

CODE_OF_CONDUCT.md

@@ -0,0 +1,32 @@
+# Contributor Covenant Code of Conduct
+
+## Our Pledge
+
+We as members, contributors, and leaders pledge to make participation in our
+community a harassment-free experience for everyone.
+
+## Our Standards
+
+Examples of behavior that contributes to a positive environment:
+
+- Demonstrating empathy and kindness
+- Being respectful of differing opinions
+- Giving and gracefully accepting constructive feedback
+- Focusing on what is best for the community
+
+Examples of unacceptable behavior:
+
+- Harassment, trolling, or insulting comments
+- Public or private harassment
+- Publishing others' private information without permission
+
+## Enforcement
+
+Project maintainers are responsible for clarifying and enforcing standards.
+Instances of abusive behavior may be reported to the maintainers via GitHub issues
+or the contact method listed in the repository.
+
+## Attribution
+
+This Code of Conduct is adapted from the [Contributor Covenant](https://www.contributor-covenant.org),
+version 2.1.

README.md

@@ -405,3 +405,7 @@ Change the attribute value in the relevant component file. Do not add `role="ale
 |-----------|---------------|
 | `frontend/src/app/(marketing)/landing/components/WaitlistHeroSection.test.tsx` | label/id pairing, `aria-required`, `aria-describedby`, persistent live region, initial `aria-invalid` state |
 | `frontend/src/components/TaskFilter.test.tsx` | all 6 `for=` → `id=` associations for inputs and selects |
+
+## Code of Conduct
+
+See [CODE_OF_CONDUCT.md](CODE_OF_CONDUCT.md).