DOCS: add growth readiness assets

[irgb]

Summary

• add contribution, security, privacy/data, launch-copy, growth-playbook, and star tracking docs
• add GitHub issue templates for bugs, feature requests, and learning-flow feedback
• add a PR template and link trust/contribution entry points from both READMEs

Validation

• ruby -ryaml -e 'ARGV.each { |p| YAML.load_file(p); puts "OK #{p}" }' .github/ISSUE_TEMPLATE/*.yml\n- docs-only change; Flutter tests not run\n\n## Notes\n\nThis supports authentic project growth by improving onboarding, feedback capture, and launch readiness. It does not automate stars or fake engagement.

[irgb]

Closing because the requested work is promotion, not repository modification.