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>

internal/tui/theme/help.go

@@ -0,0 +1,15 @@
+package theme
+
+import "github.com/charmbracelet/bubbles/help"
+
+// NewHelp returns a help renderer in the theme's colors.
+func NewHelp(t Theme) help.Model {
+	h := help.New()
+	h.Styles.ShortKey = Fg(t.Text).Bold(true)
+	h.Styles.ShortDesc = Fg(t.Muted)
+	h.Styles.ShortSeparator = Fg(t.Border)
+	h.Styles.FullKey = h.Styles.ShortKey
+	h.Styles.FullDesc = h.Styles.ShortDesc
+	h.Styles.FullSeparator = h.Styles.ShortSeparator
+	return h
+}