add CODE_OF_CONDUCT.md

CODE_OF_CONDUCT.md

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+# Contributor Covenant
+
+We're here to build something useful.
+
+This project is open to anyone who wants to help, regardless of experience
+level, background, or preferred text editor. All we ask is that you treat
+others the way you'd want to be treated.
+
+**Be constructive.** Critique code, not people. Assume good intent.
+Ask questions before making assumptions. If something can be interpreted
+as helpful or hostile, we assume helpful.
+
+**Be direct but civil.** "This approach has a race condition" is useful.
+"This is stupid" is not. We're all learning — some of us in public.
+
+**Respect boundaries.** Not everyone wants unsolicited advice on their
+coding style. Not everyone works the same hours or at the same pace.
+Give people space to respond.
+
+**No drama.** If you have a problem with someone, address it privately
+and respectfully. If that doesn't work, escalate to the maintainer.
+
+This is a small project. The maintainer reserves the right to remove
+comments, close issues, or block users who make the space unpleasant
+for others. This power is used sparingly and with explanation.
+
+That's it. Be decent. Build things.