Background process management for Pi. Start long-running commands without blocking the conversation.

chore(lint): ban pi-tui truncateToWidth with a Biome plugin master

Add plugins/no-pi-tui-truncate.grit and register it in biome.json so importing truncateToWidth from @earendil-works/pi-tui fails the lint. Document the untrusted-text rendering rules in AGENTS.md alongside it.