dave: skip cwd folder header when only one session in that worktree

The session-list sidebar wraps every cwd in a collapsible folder, even when that cwd contains only a single agent session. With one session there's nothing to collapse, so the folder is just visual noise — the user has to click into the folder to see the agent's path, and the disclosure triangle does nothing useful. ## Proposed behavior - When a cwd has exactly one session, render the session row inline (no folder header) with the cwd path drawn beneath the title — matching the layout that existed before #1454 introduced grouped folders. - When a cwd has two or more sessions, keep the collapsible folder header. - `SessionManager::visual_order` should ignore `is_cwd_collapsed` for single-session cwds, so a stale collapse-state entry can't desync keyboard navigation from what's visually rendered. Picked up while working on #1454.