feat: add CI/CD and release automation

- Add GitHub Actions CI for cross-platform testing (ubuntu/windows/macos × node 18/20/22)
- Add release workflow that auto-creates GitHub Releases and publishes to npm on tag push
- Add CONTRIBUTING.md with branching strategy (maintainers direct commit, contributors PR)
- Add MAINTAINERS.md with release workflows and recovery procedures
- Add PR template for contributors

Closes #221

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/pull_request_template.md

@@ -0,0 +1,24 @@
+## What
+
+<!-- One sentence: what does this PR do? -->
+
+## Why
+
+<!-- One sentence: why is this change needed? -->
+
+## Testing
+
+- [ ] Tested on macOS
+- [ ] Tested on Windows
+- [ ] Tested on Linux
+
+## Checklist
+
+- [ ] Follows GSD style (no enterprise patterns, no filler)
+- [ ] Updates CHANGELOG.md for user-facing changes
+- [ ] No unnecessary dependencies added
+- [ ] Works on Windows (backslash paths tested)
+
+## Breaking Changes
+
+None