Fix CI checkbox lands in PR Walkthrough

Fix CI checkbox |

CodeRabbit can now show a Fix failing CI checkbox in the ✨ Finishing Touches section of the PR Walkthrough when it detects failing CI checks. The checkbox starts the same remediation flow as @coderabbitai fix-ci: CodeRabbit investigates the failures in a sandbox and opens a stacked pull request with proposed fixes.

See the Fix CI failures documentation for details.