Establish repository security and quality safeguards

[Perinban]

Completed on July 19, 2026.

Scope:

• Added SECURITY.md with vulnerability reporting guidance
• Added weekly Dependabot checks for Python and GitHub Actions
• Added CodeQL scanning for Python
• Verified CI on Python 3.12 and 3.13
• Updated GitHub Actions dependencies in one maintenance commit
• Confirmed no exposed secrets in tracked files or history

This issue records the initial security and quality baseline for the repository.

[Perinban]

Completed as part of the repository security and quality baseline on July 19, 2026.

[Perinban]

Related dependency-update PRs: #9, #10, and #11. They were reviewed, validated by CI and CodeQL, then consolidated into the single commit d505707 to keep the project history clean.