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/keys.go

@@ -26,6 +26,7 @@ type keyMap struct {
 	Cards   key.Binding
 	Sort    key.Binding
 	Intent  key.Binding
+	Absorb  key.Binding
 }
 
 var keys = keyMap{
@@ -52,4 +53,5 @@ var keys = keyMap{
 	Cards:   key.NewBinding(key.WithKeys("2"), key.WithHelp("2", "cards")),
 	Sort:    key.NewBinding(key.WithKeys("s"), key.WithHelp("s", "sort")),
 	Intent:  key.NewBinding(key.WithKeys("tab"), key.WithHelp("tab", "intent")),
+	Absorb:  key.NewBinding(key.WithKeys("m"), key.WithHelp("m", "absorb")),
 }