* feat(session): surface background shell and subagent work

* fix(session): address review feedback on background work UI

…361)

* feat(sidebar): add repo-grouped session navigation with live status

- Add a searchable, collapsible session tree to the desktop sidebar, grouped by repository with pinned and time-bucketed ordering.
- Track per-session busy, retry, and compact status in a shared store fed by the SSE event stream and scoped by directory.
- Extract shared navigation components reused by the desktop tree and the mobile repo quick-switch sheet.
- Show a Git tab and hide Schedules inside a repo on the mobile tab bar; add Repos and Assistant entries to the session More drawer.
- Replace the CommitsTab local relative-time formatter with a shared formatShortRelativeTime util.

* refactor(sidebar): unify session status tracking and navigation helpers

* feat(sidebar): add collapsible sessions and menu sections

* fix(sessions): keep specific live status over snapshot busy

Replace the independent per-section collapse state with an accordion hook so opening Sessions or Menu closes the other.

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