docs(contributing): add CONTRIBUTING, code of conduct and issue/PR templates

Add CONTRIBUTING.md covering all validation gates and contribution
workflows, CODE_OF_CONDUCT.md (Contributor Covenant 2.1), GitHub issue
templates (bug/feature), and a pull request template; add a Contributing
section to the README.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+## What does this PR do?
+
+<!-- One or two sentences: what changed and why. -->
+
+## Validation gates
+
+<!-- Mark what you ran and that it passed. CI runs all of these on the PR too. -->
+
+- [ ] `python3 scripts/test-lint-a11y.py`
+- [ ] `python3 scripts/lint-skin.py --all --baseline`
+- [ ] `python3 scripts/verify-sequence-oauth.py`
+- [ ] `python3 scripts/verify-drawio-import.py`
+- [ ] `python3 scripts/verify-mermaid-import.py`
+- [ ] `git diff --exit-code` green after `python3 scripts/build-icons.py` (if icons changed)
+- [ ] Docs updated in the same PR where behavior changed
+
+## Checklist
+
+- [ ] No new entries added to `scripts/lint-skin-baseline.txt`
+- [ ] Generated and source files are consistent (extractor ↔ verifier ↔ reference ↔ command)
+- [ ] Accessible SVG contract satisfied for new/changed examples
+- [ ] Code of Conduct respected
+
+## Related issues
+
+<!-- Link any issues this closes, e.g. "Closes #12". -->