fix(sidebar): Unassigned chip resolves child projects through reference rows

The chip check now reuses the ancestor map built by _partitionSidebarSessionRows,
so a child whose project parent is only a hidden reference row is not counted
as unassigned. Document project inheritance for subagent sessions in README.

README.md

@@ -220,7 +220,7 @@ If an AI assistant is helping with install, reinstall, bootstrap, provider setup
 - Session actions via `⋯` dropdown per session — pin, move to project, archive, duplicate, delete
 - Pin/star sessions to the top of the sidebar (gold indicator)
 - Archive sessions (hide without deleting, toggle to show)
-- Session projects -- named groups with colors for organizing sessions
+- Session projects -- named groups with colors for organizing sessions; delegated subagent sessions have no project of their own and follow their nearest ancestor's project in the project filter and the Unassigned chip
 - Session tags -- add #tag to titles for colored chips and click-to-filter
 - Grouped by Today / Yesterday / Earlier in the sidebar (collapsible date groups)
 - Download as Markdown transcript, full JSON export, or import from JSON