ci: CodeQL, split secret-scan workflow, status badges

[sumitake]

Closes the security-visibility gaps:

• CodeQL: weekly + per-PR Python analysis, adapted from the Portable-GHAR house template (pinned action SHAs, minimal permissions)
• secret-scan split into its own workflow so it gets a distinct badge; job name unchanged, so the branch-protection required check is unaffected
• README badges: ci, CodeQL, secret-scan, license
• (Applied directly via API, not in this PR: GitHub native secret scanning + push protection are now enabled on the repo — Security tab)

🤖 Generated with Claude Code

[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.