Exclude sample/ from CodeQL analysis

[Kristof Roomp (mcroomp)]

Summary

• Adds .github/codeql/codeql-config.yml to exclude the sample/ fixture directory from CodeQL scanning
• The sample/ directory contains intentionally minimal test fixtures, not production code, so CodeQL alerts from it are noise

Test plan

• Verify CodeQL re-runs after merge and alert count drops for issues originating in sample/

🤖 Generated with Claude Code

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