Remove thread parenting

[ymichael]

Summary

• remove persisted thread parent relationships, parent-aware DB helpers, public API fields/routes, SDK/CLI flags, and thread parenting errors
• delete server parent/child lifecycle services, parent notification templates, child confirmation, archive-all behavior, and child-thread telemetry
• flatten app thread UI/cache behavior by removing parent selectors, child summaries, parent banners, archived root/child filters, and parent-change timeline rows

Validation

• pnpm exec turbo run typecheck --filter=@bb/db --filter=@bb/domain --filter=@bb/server-contract --filter=@bb/thread-view --filter=@bb/server --filter=@bb/sdk --filter=@bb/cli --filter=@bb/app --filter=@bb/templates --filter=@bb/scripts
• pnpm exec turbo run test --filter=@bb/db --filter=@bb/server-contract --filter=@bb/thread-view --filter=@bb/server --filter=@bb/cli --filter=@bb/app --filter=@bb/scripts > /tmp/remove-thread-parenting-tests.txt 2>&1