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/architecture.md

@@ -37,7 +37,7 @@ internal/tui/                 Bubble Tea application
   detail/                     detail pane; changes.go: change preview
   header/                     header line
   branches/                   branch picker popup (search line + list)
-  footer/                     key bar (navigation left, global right) and flash messages
+  footer/                     key bar (navigation and tools left, global right), full help pages (keys, legend) and flash messages
 ```
 
 ### Layering rules