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/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,8 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/kingchenc/wickra/security/advisories/new
+    about: Report security issues privately — do not open a public issue.
+  - name: Question or discussion
+    url: https://github.com/kingchenc/wickra/discussions
+    about: Ask usage questions and discuss ideas here.