feat(tui): add tag autocomplete and query composition

Tag autocomplete shows suggestions when typing #partial in capture bar.
Tab/enter accepts, up/down navigates, esc dismisses.

Query composition extends ? search with date filters (@today, @week,
@month, <7d, >30d), card type filters (^snippet), all composable
with existing text and tag filters.

internal/tui/help.go

@@ -18,10 +18,18 @@ func renderHelp(width, height int) string {
 		{"Capture Bar", [][2]string{
 			{"enter", "submit (or browse if empty)"},
 			{"?…", "search (type ?query)"},
+			{"#…", "tag (autocomplete with tab)"},
 			{"-", "todo prefix"},
 			{"@", "event prefix"},
 			{"!", "reminder prefix"},
 		}},
+		{"Query Operators", [][2]string{
+			{"?text", "substring search"},
+			{"?#tag1 #tag2", "filter by tags (AND)"},
+			{"?@today @week", "date filter (@yesterday @month)"},
+			{"?<7d >30d", "newer/older than N days"},
+			{"?^snippet", "card type filter"},
+		}},
 		{"Navigation", [][2]string{
 			{"j/k ↑/↓", "move cursor"},
 			{"g/G home/end", "top / bottom"},