Harden dependency and security workflows

[FU-max-boop]

Summary

• add weekly Dependabot version updates for Python and GitHub Actions
• exclude immutable hash-locked history and upstream evidence inputs from pip updates
• add full-SHA-pinned Python CodeQL analysis for pull requests, main pushes, and a weekly schedule
• link private vulnerability reporting and publish response targets without presenting them as guarantees

Security boundaries

• CodeQL has contents: read and job-scoped security-events: write; checkout does not persist credentials.
• Dependabot keeps major updates separate for review and limits each ecosystem to three open version-update PRs.
• This does not enable Dependabot security updates or change repository rulesets, permissions, or secrets.

Validation

• actionlint v1.7.12 passed after checksum verification
• all GitHub YAML parsed successfully
• Dependabot v2 schema validation passed
• verified official action tags and exact commits:
  • actions/checkout v7.0.1 → 3d3c42e5aac5ba805825da76410c181273ba90b1
  • github/codeql-action v4.37.7 → ff2f1c621b7f889edc0d3c761ac2e6a3f8cdb0dd
• Ruff check/format and mypy passed
• pytest: 170 passed, 1 skipped
• git diff --check passed

[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.