feat(tui-go): idea board viewer

OverlayIdeas (opened with I, global/cross-session) lists the idea board in the
artifact-viewer shell: ↑↓ to move, x/d to remove (sends DiscardIdea, optimistic
drop), esc to close. Pull-based via ListIdeas -> idea.list.

apps/tui-go/internal/app/update.go

@@ -176,6 +176,8 @@ func (m Model) handleNormalKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		m.overlay = OverlayToolPalette
 	case "v":
 		m.openArtifacts()
+	case "I":
+		m.openIdeas()
 	case "S":
 		m.openStats()
 	case "g":
@@ -442,6 +444,28 @@ func (m Model) handleOverlayKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		if runeIs(k, "S") {
 			m.overlay = OverlayNone
 		}
+	case OverlayIdeas:
+		switch {
+		case k.Type == tea.KeyUp || runeIs(k, "k"):
+			if m.ideasIndex > 0 {
+				m.ideasIndex--
+			}
+		case k.Type == tea.KeyDown || runeIs(k, "j"):
+			if m.ideasIndex < len(m.ideas)-1 {
+				m.ideasIndex++
+			}
+		case runeIs(k, "x") || runeIs(k, "d"):
+			if m.ideasIndex >= 0 && m.ideasIndex < len(m.ideas) {
+				m.client.Send(protocol.DiscardIdea(m.ideas[m.ideasIndex].IdeaID))
+				// Optimistic removal; the server's fresh idea.list reply reconciles.
+				m.ideas = append(m.ideas[:m.ideasIndex], m.ideas[m.ideasIndex+1:]...)
+				if m.ideasIndex >= len(m.ideas) && m.ideasIndex > 0 {
+					m.ideasIndex--
+				}
+			}
+		case runeIs(k, "I"):
+			m.overlay = OverlayNone
+		}
 	}
 	return m, nil
 }
@@ -463,6 +487,17 @@ func (m *Model) openArtifacts() {
 	m.client.Send(protocol.ListArtifacts(m.selectedID))
 }
 
+// openIdeas opens the cross-session idea board and requests it from the server. The board
+// is global (not session-scoped), so it opens from anywhere — including the idle list.
+func (m *Model) openIdeas() {
+	m.overlay = OverlayIdeas
+	m.ideasIndex = 0
+	if m.ideas == nil {
+		m.ideasLoading = true
+	}
+	m.client.Send(protocol.ListIdeas())
+}
+
 // openStats opens the session-stats pane for the selected session and requests its
 // metrics from the server. No-op when no session is selected.
 func (m *Model) openStats() {