[codex] Show assigned managers in sidebar

[brsbl]

Summary

• Treat manager threads as valid children of other managers in sidebar grouping.
• Render nested manager groups recursively in project and pinned sidebar trees.
• Make pinned manager roots collect assigned manager descendants and hide pinned descendants under their pinned ancestor.

Root Cause

Sidebar grouping only considered standard threads as manager children. In the pinned sidebar, an explicitly pinned manager assigned to a pinned manager was removed from root items but never added under the parent group, so it disappeared.

Validation

• pnpm exec turbo run test --filter=@bb/app -- --run src/components/sidebar/projectThreadGroups.test.ts src/components/sidebar/pinnedSidebarThreads.test.ts
• pnpm exec turbo run typecheck --filter=@bb/app