Make the screen explain itself: labels, tools, help pages, hints

For developers new to submodules (notes/usability-plan.md, D32):
- x and X read "restore to pin" and "restore committed pins"
- problems only, filter and fetch are shown in the footer
- ? shows every key in titled columns, then a legend of the status
  words, symbols and spine roles; esc closes it
- u reads "pull (fast-forward)" on the superproject
- the detail pane suggests f when the last fetch is old
- a hint above the footer until the first key
- the header says "staged", like the tree

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/README.md

@@ -42,3 +42,7 @@ Next steps, roughly in order:
 1. (done) Write actions: restore, switch branch, update pin, see
    [actions.md](actions.md). Stage pin was dropped (D29).
 2. (done) "Problems only" (`!`) view, reusing the tree's filter.
+3. Usability pass for users new to submodules, see
+   [notes/usability-plan.md](../notes/usability-plan.md). The low-effort
+   items are done (D31, D32); next are the issue hints and the palette
+   that fits the selection.