Show idle state in the top tab bar

Each agent tab now prefixes its label with the same one-rune idle
indicator the sidebar uses (✕ error, ? permission, ◐ thinking, ○ idle,
● working), so the state of every open agent is visible without
opening or focusing each tab. Tab redraws now fire on idle-state
changes in addition to sidebar redraws.

internal/app/app.go

@@ -829,11 +829,11 @@ func (st *uiState) OnChildSpawned(c *Child) {
 	st.drawStatusLine()
 }
 
-// OnChildStateChanged repaints the sidebar whenever a child's
-// idle-state badge flips. Cheap — the badge is the only chrome that
-// reflects state today, and drawSidebar bails when the cached frame
-// hasn't changed.
+// OnChildStateChanged repaints the sidebar and tab bar whenever a
+// child's idle-state badge flips. Cheap — both draws bail when the
+// cached frame hasn't changed.
 func (st *uiState) OnChildStateChanged(string, IdleState) {
+	st.drawTabBar()
 	st.drawSidebar()
 }