feat(tui): add search via capture bar and absorb flow

Search uses existing parse grammar ?prefix — type `?query #tag` in
capture bar to filter entities client-side. Substring match on
body+title+description with AND tag filtering. Esc clears search.

Absorb via m key on fluid entities — opens source picker showing all
other entities, enter merges source into target. Uses existing
store.Absorb() backend.

internal/tui/help.go

@@ -21,11 +21,12 @@ func renderHelp(width, height int) string {
 			{"tab", "cycle intent (cards)"},
 		}},
 		{"Actions", [][2]string{
-			{"a", "add entity"},
+			{"a", "add entity (or ?query to search)"},
 			{"d", "delete (with confirm)"},
 			{"x", "toggle todo completion"},
 			{"!", "toggle pin"},
 			{"#", "filter by tag"},
+			{"m", "absorb (merge into target)"},
 			{"p", "promote to card"},
 		}},
 		{"Detail View", [][2]string{