fix: ignore reaction tags inside fenced code

[vycdev2]

Summary

• Keep literal [REACT:emoji] text inside fenced code blocks as user-facing content.
• Preserve existing reaction directives outside fenced code blocks.

Verification

• RED: node --test --test-concurrency=1 tests/responseCodeFence.test.mjs failed on untouched main before the implementation.
• GREEN: focused code-fence regression and tests/response.test.mjs passed after the implementation.
• npm ci passed.
• npm run build passed.
• npm test passed.
• Direct node-pty import passed.
• git diff --check and staged diff checks passed.

Risk

The parser now masks fenced CommonMark code blocks before interpreting reaction directives, without changing text output.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.