Five spec-fidelity fixes against pi's rpc.md:
- a session_before_switch extension veto now fails the resume instead of
  silently adopting whichever session stayed active
- failed compactions surface as a failed compaction item with the error
  instead of disappearing
- empty input/editor dialog answers deliver as values (the spec's
  'extension receives ""'), no longer converted to cancels
- editor dialogs show their prefill inside the question text so the user
  is not editing blind
- text generation passes --no-extensions so an unanswerable extension
  dialog cannot stall commit-message generation until its timeout

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>