feat(maestro): keyboard navigation for the web sidebar

Ctrl+Shift+H moves the keyboard between the session list and the pane;
j/k walk the cards, Enter opens, Escape hands the pane back. The walk
skips expanded folder headers and stops on collapsed ones, matching the
TUI.

docs/SIDEBAR.md

@@ -146,3 +146,22 @@ dropping the whole session would be worse than dimming one card. A report
 arriving at `POST /api/sessions/:id/activity` is parsed strictly and an unknown
 state is a 4xx: an agent posting a state the daemon does not know is a bug in
 the agent, and answering 204 would hide it.
+
+## Keyboard walk
+
+`j`/`k` (and the arrow keys) move a highlight through the sidebar. Both clients
+follow one rule for what the walk stops on:
+
+- **Session cards, always.**
+- **Folder headers, only while collapsed.** An expanded folder's cards are one
+  row away, so its header is nothing to land on; a collapsed folder's header is
+  the only row it has, and skipping it would leave the folder unreachable.
+
+Enter opens the highlighted session, or expands the highlighted folder and
+lands on the first row it was hiding. A highlight parked on a skipped row —
+reachable by clicking a folder header — still steps to its neighbour rather
+than jumping to the end of the list.
+
+The web client needs a chord to reach the list at all (`Ctrl+Shift+H`, and the
+same chord back): an attached terminal owns every plain keystroke. The TUI uses
+`ctrl-w h`.