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/model.go

@@ -52,6 +52,7 @@ const (
 	OverlayArtifacts
 	OverlayConfig
 	OverlayStats
+	OverlayIdeas
 )
 
 // RouterEntry is one line in a session's conversation transcript.
@@ -253,6 +254,11 @@ type Model struct {
 	statsFor     string // sessionId the current stats belong to
 	statsLoading bool
 
+	// idea board (OverlayIdeas) — cross-session, populated by the idea.list reply
+	ideas        []protocol.IdeaDto
+	ideasIndex   int
+	ideasLoading bool
+
 	// command palette
 	paletteFilter string
 	paletteIndex  int