docs: add release checklist

docs/RELEASE_CHECKLIST.md

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+# OhMyDB Release Checklist
+
+Use this checklist before publishing a new stable OhMyDB release.
+
+## Code quality
+
+- [ ] Focused tests pass
+- [ ] Full test suite passes
+- [ ] `python -m compileall -q sql_safety_proxy` passes
+- [ ] `python -m pip check` passes
+- [ ] `git diff --check` passes
+
+## Compatibility
+
+- [ ] `ohmydb --version` works
+- [ ] Legacy `sql-safety-proxy --version` works
+- [ ] PostgreSQL path is validated
+- [ ] MySQL/MariaDB path is validated when affected
+- [ ] Supported Python versions pass CI
+
+## Packaging
+
+- [ ] Wheel builds successfully
+- [ ] Source distribution builds successfully
+- [ ] `twine check` passes
+- [ ] Fresh virtual-environment install succeeds
+
+## Docker
+
+- [ ] Image builds successfully
+- [ ] CLI works inside the image
+- [ ] Container runs as a non-root user
+
+## Release
+
+- [ ] Version references are updated
+- [ ] Release notes summarize user-visible changes
+- [ ] Release tag points to the intended `main` commit
+- [ ] Release assets are uploaded
+- [ ] SHA-256 checksums are generated
+- [ ] GitHub release is marked stable when appropriate
+
+## Final verification
+
+After publishing, verify the release from a clean environment instead of relying only on the development checkout.