Closes the remaining coverage gaps against pi's RPC surface:

- forkThread clones pi's session tree and switches the live process back
  to the source, so T3-side thread forks map to native clones.
- readThreadSnapshot reconstructs the conversation from get_entries,
  giving handoff and preview surfaces a real transcript.
- setStatus/setWidget extension calls project as keyed live work-log
  rows that update in place and close on settle.
- Session-start extension dialogs (e.g. project trust) are buffered and
  attached to the next turn instead of being cancelled unseen.
- Stop-with-restart aborts the turn and terminates the pi process, and
  transport death during an interrupt reads as interrupted, not failed.
- A recovered auto-compaction clears the stashed model error so the
  turn completes instead of reporting the pre-compaction failure.
- Opening a subagent's child thread while its task is still running is
  refused with a clear message instead of corrupting the child session.

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