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/protocol.go

@@ -54,6 +54,7 @@ const (
 	TypeArtifactList     = "artifact.list"
 	TypeConfigSnapshot   = "config.snapshot"
 	TypeSessionStats     = "session.stats"
+	TypeIdeaList         = "idea.list"
 )
 
 // ServerMessage is a flat decode of every server->client variant. Field names
@@ -146,6 +147,16 @@ type ServerMessage struct {
 	Prompt          string                `json:"prompt"`
 	OriginalRequest string                `json:"originalRequest"`
 	Candidates      []ProposedWorkflowDto `json:"candidates"`
+
+	// idea.list — the cross-session idea board
+	Ideas []IdeaDto `json:"ideas"`
+}
+
+// IdeaDto is one idea on the cross-session board. Mirrors the Kotlin IdeaDto.
+type IdeaDto struct {
+	IdeaID       string `json:"ideaId"`
+	Text         string `json:"text"`
+	CapturedAtMs int64  `json:"capturedAtMs"`
 }
 
 // ProposedWorkflowDto is one candidate workflow the router suggests, with a short
@@ -385,6 +396,16 @@ func GetConfig() []byte {
 	return encode("GetConfig", map[string]any{})
 }
 
+// ListIdeas requests the cross-session idea board (replied to with an idea.list).
+func ListIdeas() []byte {
+	return encode("ListIdeas", map[string]any{})
+}
+
+// DiscardIdea removes an idea from the board (server tombstones it; replies with a fresh idea.list).
+func DiscardIdea(ideaID string) []byte {
+	return encode("DiscardIdea", map[string]any{"ideaId": ideaID})
+}
+
 // UpdateConfig asks the server to apply and persist a config patch (key -> string value).
 func UpdateConfig(patch map[string]string) []byte {
 	return encode("UpdateConfig", map[string]any{"patch": patch})