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>

README.md

@@ -168,15 +168,15 @@ files.
 | `H` `L` / `⇧←` `⇧→` | Collapse / expand all |
 | `space` | Toggle the selected submodule |
 | `home` `end` / `g` `G`, `pgup` `pgdn` | Jump, page |
-| `/` | Filter |
 | `!` | Problems only |
+| `/` | Filter |
 | `c` | Copy path |
 | `u` | Update: move a pin that is behind to the tip of its branch (staged, like `b`); on the header, fast-forward the superproject |
 | `b` | Switch branch: pick a local or remote branch; its tip is staged and the repository restored to it |
-| `x` | Restore: make the worktree exactly what git records (asks if work would be lost) |
-| `X` | Hard restore: like `x`, but to the committed pins, dropping all staged pins below |
+| `x` | Restore to pin: make the worktree exactly what git records (asks if work would be lost) |
+| `X` | Restore committed pins: like `x`, but dropping all staged pins below (asks first) |
 | `f` / `r` | Fetch / reload |
-| `?` | Full help |
+| `?` | Full help: every key, then what the words and symbols mean (`esc` closes it) |
 | `q` | Quit. If a newer release exists, the footer offers `q quit and update to vX` (highlighted) and plain quit moves to `Q` |
 
 The mouse works too: click to select, click `▸` to toggle, wheel to move. In