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/protocol/protocol.go

@@ -35,6 +35,7 @@ const (
 	TypeApprovalRequired = "approval.required"
 	TypeApprovalResolved = "approval.resolved"
 	TypeClarification    = "clarification.required"
+	TypeWorkflowProposed = "workflow.proposed"
 	TypeWorkspaceBound   = "session.workspace_bound"
 	TypeStageManifest    = "stage.tool_manifest"
 	TypeProviderStatus   = "provider.status_changed"
@@ -139,6 +140,19 @@ type ServerMessage struct {
 
 	// clarification.required — open questions a stage raised for the operator
 	Questions []ClarificationQuestionDto `json:"questions"`
+
+	// workflow.proposed — the router's triage suggestion of candidate workflows
+	ProposalID      string                `json:"proposalId"`
+	Prompt          string                `json:"prompt"`
+	OriginalRequest string                `json:"originalRequest"`
+	Candidates      []ProposedWorkflowDto `json:"candidates"`
+}
+
+// ProposedWorkflowDto is one candidate workflow the router suggests, with a short
+// reason. Mirrors the Kotlin ProposedWorkflow (core:events), the wire shape.
+type ProposedWorkflowDto struct {
+	WorkflowID string `json:"workflowId"`
+	Reason     string `json:"reason"`
 }
 
 // ClarificationQuestionDto is one open question from a stage. Empty Options ->
@@ -313,6 +327,7 @@ func (m ServerMessage) IsEventBearing() bool {
 		TypeToolStarted, TypeToolCompleted, TypeToolFailed, TypeToolRejected,
 		TypeToolAssessed,
 		TypeApprovalRequired, TypeApprovalResolved, TypeClarification,
+		TypeWorkflowProposed,
 		TypeWorkspaceBound, TypeArtifactCreated,
 		TypeArtifactValid, TypePlanLocked,
 		TypeRouterNarration: