Harden dependency and workflow posture

[langsmith-fleet]

Summary

• Add Dependabot coverage for GitHub Actions and uv dependencies
• Add CodeQL and zizmor workflows for Python and GitHub Actions security scanning
• Fix the reusable test workflow so the Python version is explicitly passed from CI
• Fix the release workflow by removing an invalid needs.release-notes reference
• Expand .gitignore coverage for common local secret files before committing

Validation

• git diff --check
• Parsed updated YAML files with Ruby YAML.load_file
• Verified .gitignore covers .env, .env.*, *.pem, *.key, *.crt, credentials.json, node_modules/, __pycache__/, and .venv
• Verified no secret-like files were staged before commit

[github-advanced-security]

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