chore: establish security automation baseline

[PureSaber]

Summary

• add weekly grouped Dependabot updates for detected package ecosystems and GitHub Actions
• enable a scheduled and PR-triggered Python CodeQL workflow
• add SECURITY.md, CODEOWNERS, issue forms, and a PR template
• enforce minimal workflow permissions, full Action SHA pins, and non-persisted checkout credentials

Verification

• YAML parsing passed
• every external Action is pinned to a 40-character SHA
• all workflows declare explicit minimal permissions
• git diff --check passed
• existing CI and CodeQL will run on this PR

[github-advanced-security]

You are seeing this message because GitHub Code Scanning has recently been set up for this repository, or this pull request contains the workflow file for the Code Scanning tool.

What Enabling Code Scanning Means:

• The 'Security' tab will display more code scanning analysis results (e.g., for the default branch).
• Depending on your configuration and choice of analysis tool, future pull requests will be annotated with code scanning analysis results.
• You will be able to see the analysis results for the pull request's branch on this overview once the scans have completed and the checks have passed.

For more information about GitHub Code Scanning, check out the documentation.