chore: add GitHub contribution templates and trim CI matrix

[guantw]

Changes

• Add structured issue forms for bug reports and feature requests
• Add a pull request checklist template
• Disable blank issues for regular contributors and direct security reports to the security policy
• Remove Python 3.12 from the CI matrix, keeping Python 3.11 and 3.13
• Keep Dependabot disabled by excluding its configuration

Validation

• git diff --check passes
• The pull request contains only the five intended files under .github
• All four CI jobs pass on Windows and macOS with Python 3.11 and 3.13
• Application code and project dependencies are unchanged