Fix Failing Tests

Find the failures in a test-output file, have a read-only subagent diagnose their root causes, then apply the fixes in the main thread, where the user can redirect or stop any fix before work piles up.

1. Find the output. Use the path the user gave, else /tmp/output. Read it; if it's missing or empty, ask the user where the output is. Done when you have a non-empty file.

2. Search for failures. Search for markers rather than reading, because failures slip past in long output mixed with passing lines and infra logs:

   rg -n 'FAIL|--- FAIL|^panic:|DATA RACE|^not ok|Error Trace|build failed' <path>
   

   If nothing matches, report "no failures found" with the command you ran, and stop. Done when you have the list of failures, or have reported none.