Migrate tool temp line and tool output

• Status: Done
• Kind: Feature
• Authors: Jean Mertz git@jeanmertz.com
• Date: 2026-08-27
• Implements: 091

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.