Add c to copy the selected path, split key bar into groups

c copies the absolute directory of the selected submodule via OSC 52.
Keys are grouped into navigation (footer left), global (footer right)
and submodule (detail pane palette, footer if the pane can't show it).
Adds a key reference table to the UX guide.

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

docs/architecture.md

@@ -31,7 +31,7 @@ internal/tui/                 Bubble Tea application
   tree/                       tree component: rows, navigation, mouse, view
   detail/                     detail pane; changes.go: change preview
   header/                     header line
-  footer/                     help and flash messages
+  footer/                     key bar (navigation left, global right) and flash messages
 ```
 
 ### Layering rules
@@ -64,6 +64,12 @@ type Watcher interface {
 }
 ```
 
+```go
+type Locator interface {
+    Dir(path string) string                    // absolute directory, "" if unknown
+}
+```
+
 - `Tree` returns the superproject and all submodules with `Parent` and
   `Children` linked. `Status` fields are nil.
 - `Status` is called once per submodule, concurrently. It must be safe to