feat(tui-go): workflow-propose choice panel

Render a workflow.proposed frame as a single-select picker with a manual-answer
slot: ↑↓/jk to choose, enter to launch, e for custom, esc to peek away. Picking
a candidate sends StartSession (and focuses the launched session); the custom
slot continues the conversation via ChatInput. Reuses the modal helpers from the
clarification view.

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

@@ -133,12 +133,14 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		if s := m.session(msg.SessionID); s != nil {
 			s.Active = false
 			s.Clar = nil
+			s.Propose = nil
 		}
 	case protocol.TypeSessionFailed:
 		m.touch(msg.SessionID, "FAILED")
 		if s := m.session(msg.SessionID); s != nil {
 			s.Active = false
 			s.Clar = nil
+			s.Propose = nil
 		}
 	case protocol.TypeStageStarted:
 		if s := m.session(msg.SessionID); s != nil {
@@ -246,6 +248,8 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 		m.onApprovalRequired(msg)
 	case protocol.TypeClarification:
 		m.onClarificationRequired(msg)
+	case protocol.TypeWorkflowProposed:
+		m.onWorkflowProposed(msg)
 	case protocol.TypeApprovalResolved:
 		if s := m.session(msg.SessionID); s != nil {
 			s.Pending = nil
@@ -424,6 +428,23 @@ func (m *Model) onClarificationRequired(msg protocol.ServerMessage) {
 	}
 }
 
+func (m *Model) onWorkflowProposed(msg protocol.ServerMessage) {
+	cands := make([]ProposeCandidate, 0, len(msg.Candidates))
+	for _, c := range msg.Candidates {
+		cands = append(cands, ProposeCandidate{WorkflowID: c.WorkflowID, Reason: c.Reason})
+	}
+	p := &Proposal{
+		ProposalID: msg.ProposalID, SessionID: msg.SessionID,
+		Prompt: msg.Prompt, OriginalRequest: msg.OriginalRequest, Candidates: cands,
+	}
+	if s := m.session(msg.SessionID); s != nil {
+		s.Propose = p
+	}
+	if msg.SessionID == m.selectedID {
+		m.proposeInitState()
+	}
+}
+
 func (m *Model) onSnapshot(msg protocol.ServerMessage) {
 	var pending *Approval
 	if len(msg.PendingAppr) > 0 {