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/protocol/golden_test.go

@@ -149,6 +149,21 @@ func TestDecodeGoldenServerFrames(t *testing.T) {
 				}
 			},
 		},
+		{
+			name: "idea.list carries the cross-session board",
+			json: `{"type":"idea.list","ideas":[{"ideaId":"i1","text":"cache the repo map","capturedAtMs":1000},` +
+				`{"ideaId":"i2","text":"add --dry-run","capturedAtMs":2000}]}`,
+			wantType:  TypeIdeaList,
+			eventBear: false,
+			check: func(t *testing.T, m ServerMessage) {
+				if len(m.Ideas) != 2 {
+					t.Fatalf("idea.list ideas: %+v", m.Ideas)
+				}
+				if m.Ideas[0].IdeaID != "i1" || m.Ideas[0].Text != "cache the repo map" || m.Ideas[0].CapturedAtMs != 1000 {
+					t.Fatalf("idea.list idea[0]: %+v", m.Ideas[0])
+				}
+			},
+		},
 		{
 			name: "workflow.proposed carries candidate workflows",
 			json: `{"type":"workflow.proposed","sessionId":"s1","proposalId":"prop-1","prompt":"Run one?",` +
@@ -233,6 +248,16 @@ func TestEncodeGoldenClientFrames(t *testing.T) {
 				}
 			},
 		},
+		{
+			name:     "DiscardIdea carries the ideaId",
+			frame:    DiscardIdea("i1"),
+			wantType: clientPrefix + "DiscardIdea",
+			check: func(t *testing.T, raw map[string]any) {
+				if raw["ideaId"] != "i1" {
+					t.Fatalf("DiscardIdea ideaId: %+v", raw)
+				}
+			},
+		},
 		{
 			name: "ClarificationResponse carries ids + answers",
 			frame: ClarificationResponse("s1", "analyst", "req-1",