fix(tui): simplify focus model — tab toggles capture ↔ list only

Tag rail removed from tab cycle to reduce focus confusion.
Rail is now ambient-by-default, focusable via h from list (spatial).

- Tab: capture ↔ list (no rail, no detail in cycle)
- h from list: focus tag rail (when visible)
- l from rail: back to list
- Split detail reachable via l/enter, not tab
- Remove nextFocusFromCapture helper

internal/tui/help.go

@@ -8,9 +8,11 @@ func renderHelp(width, height int) string {
 		binds [][2]string
 	}{
 		{"Focus", [][2]string{
-			{"tab", "cycle focus: capture → tags → list → detail"},
+			{"tab", "toggle capture ↔ list"},
 			{"esc", "back / clear filter / to capture"},
 			{"a", "focus capture bar"},
+			{"h", "focus tag rail (from list)"},
+			{"l", "focus detail (split view)"},
 			{"ctrl+b", "toggle tag rail"},
 		}},
 		{"Capture Bar", [][2]string{