docs: update all markdown documentation
- README.md: Update tech stack versions, simplify content, add links to docs
- CONTRIBUTING.md: Add commit guidelines, testing instructions, PR process
- SECURITY.md: Add security measures, vulnerability reporting process
- PULL_REQUEST_TEMPLATE.md: Add conventional commit types, breaking change section
- Remove outdated plan documents (already implemented or superseded)
- architecture.md: Already updated with comprehensive system documentation
- release-engineering.md: Already updated with current pipeline status