Strengthen review prompt: investigation-first, confirm-before-reporting

Three changes based on FN/FP analysis (F1=55.4% baseline):

1. Review prompt: replace generic "use tools" with phased investigation
   (understand → investigate impact → confirm before reporting)
2. Add "don't report unverified suspicions" to NOT-report list
3. WarpGrep description: distinguish semantic queries from grep lookups,
   emphasize concurrency tracing and framework API semantics