/diagnose — Structured differential diagnosis

Investigate a bug systematically by gathering evidence, forming multiple hypotheses, and testing each before proposing any fix. This prevents the "guess and fix" anti-pattern.

Steps

1. Gather the symptom from $ARGUMENTS or ask the user. Document:

   • What's happening (the observable failure)
   • What's expected (the correct behavior)
   • When it started (if known)
   • Any user-provided log evidence

2. Collect evidence — read logs, error output, and relevant source code:

   • Read the FULL log, not just the error line
   • Search for related error patterns in the codebase
   • Check docs/LINEAR_SNAPSHOT.md for related tracked bugs
   • Check recent git changes: git log --oneline -20 and git diff HEAD~5