Show every agent tab's summary, not just the focused one

The tab bar's row-2 summary was painted only for the active tab. Add a
per-child summaryTextFor/summaryRawFor helper (active variants now
delegate to it), carry each tab's childID on its tabRect, and loop over
all visible tabs so each renders its own summary under its column.
Layout is unchanged (still 3 rows); narrow tabs clip as before.

Resolves the per-tab summary TODO item.

internal/app/tabbar.go

@@ -59,6 +59,7 @@ func (st *uiState) drawTabBar() {
 	newHintW := utf8.RuneCountInString(newHint) + 2 // " + new " framing
 
 	type tabRect struct {
+		childID    string
 		startCol   int
 		width      int
 		label      string
@@ -66,8 +67,6 @@ func (st *uiState) drawTabBar() {
 		glyphStyle string
 		active     bool
 	}
-	activeTab := -1
-
 	// Reserve space at the right edge for "+ new". If there are too
 	// many tabs to fit even at minTabWidth, drop tabs from the right
 	// until they do. The current focus stays visible.
@@ -139,6 +138,7 @@ func (st *uiState) drawTabBar() {
 				labelW = utf8.RuneCountInString(label)
 			}
 			tabs = append(tabs, tabRect{
+				childID:    c.ID,
 				startCol:   col,
 				width:      w,
 				label:      label,
@@ -146,9 +146,6 @@ func (st *uiState) drawTabBar() {
 				glyphStyle: glyphStyle,
 				active:     active,
 			})
-			if tabs[len(tabs)-1].active {
-				activeTab = len(tabs) - 1
-			}
 			col += w
 		}
 	}
@@ -224,10 +221,9 @@ func (st *uiState) drawTabBar() {
 			hintCol, styleBorder, strings.Repeat("─", newHintW), styleReset)
 	}
 
-	if activeTab >= 0 {
-		tab := tabs[activeTab]
+	for _, tab := range tabs {
 		summaryWidth := tab.width - 2
-		if summary := st.activeSummaryText(summaryWidth); summary != "" {
+		if summary := st.summaryTextFor(tab.childID, summaryWidth); summary != "" {
 			fmt.Fprintf(&b, "\x1b[2;%dH %s%s%s", tab.startCol, styleDim, summary, styleReset)
 		}
 	}