Polish repository for public contributions

[vwall]

Summary

• remove internal planning history, RFCs, generated artifacts, and unused images
• streamline public documentation and route Pages links to rendered GitHub Markdown
• add pull-request CI, Dependabot, issue forms, and a pull request template
• pin GitHub Actions to immutable SHAs and add formatting to the release gate
• correct vulnerability reporting so sensitive reports stay private

Repository settings applied separately

• added a public description and topics
• enabled private vulnerability reporting, Dependabot alerts/security updates, secret scanning, and push protection
• made the default Actions token read-only
• enabled basic main protection without a required check until this CI workflow establishes its check name

Verification

• make release-check VERSION=2.7.0
• YAML syntax validation for workflows, Dependabot, and issue forms
• dangling-reference and whitespace checks
• desktop and 390px responsive browser review
• source-command copy control, CSS, favicon, and rendered documentation links