Migrate tool temp line and tool output
Done2026-08-27Jean Mertz
Migrates ToolRenderer's preparing line and execution-progress ticker to the printer, deleting spawn_tick_sender and the manual rewrite/clear machinery. Adds a line channel to forward_stderr, has the coordinator own one sink per executing tool, and carries the RFD 095 reasoning-region background across the migrated rows including the worker's own erases.