Harden linked child orchestration

[Pizzaface]

Summary

• remove the default manual session messaging path and make spawn_session always create linked child sessions
• harden linked parent/child recovery and broken-link surfacing across reconnects, relay routing, and trigger delivery
• update prompts, docs, and regression tests for the linked-only multi-agent model

Test Plan

• bun run typecheck
• bun run test
• cd packages/docs && bun run build