E3: add community health files

The repository had no contributor-facing documentation or automation
config. Add the standard set:

- CHANGELOG.md (Keep a Changelog format, 0.1.0-0.1.4 plus Unreleased)
- CONTRIBUTING.md (build/test steps, change standards, PolyForm-NC note)
- SECURITY.md (private reporting, supported versions)
- CODE_OF_CONDUCT.md (Contributor Covenant 2.1)
- .github/ISSUE_TEMPLATE (bug report, feature request, config)
- .github/PULL_REQUEST_TEMPLATE.md
- .github/dependabot.yml (cargo, npm, pip, github-actions — the last
  keeps the D1 SHA pins current)
- .github/CODEOWNERS

.github/CODEOWNERS

@@ -0,0 +1,6 @@
+# Code owners for Wickra.
+#
+# The owner listed here is requested for review automatically on every pull
+# request. See https://docs.github.com/articles/about-code-owners.
+
+*       @kingchenc