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/statusbar.go

@@ -88,7 +88,7 @@ func contextHints(m model) []hint {
 	case focusCapture:
 		return []hint{{"enter", "submit"}, {"esc", "browse"}, {"?…", "search"}, {"-", "todo"}, {"@", "event"}}
 	case focusTagRail:
-		return []hint{{"j/k", "nav"}, {"enter", "filter"}, {"ctrl+b", "hide"}, {"tab", "list"}, {"esc", "list"}}
+		return []hint{{"j/k", "nav"}, {"enter", "filter"}, {"l", "list"}, {"ctrl+b", "hide"}}
 	case focusDetail:
 		if m.splitDetail {
 			return []hint{{"h", "list"}, {"c", "copy"}, {"e", "edit"}, {"p", "promote"}, {"!", "pin"}, {"tab", "capture"}}