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/model.go

@@ -13,6 +13,7 @@ const (
 	StateInSession
 	StateApproval
 	StateClarification
+	StateWorkflowPropose
 )
 
 // InputMode toggles what a submitted line targets.
@@ -125,6 +126,23 @@ type Clarification struct {
 	Questions []ClarQuestion
 }
 
+// ProposeCandidate is one workflow the router suggests (mirrors the wire DTO).
+type ProposeCandidate struct {
+	WorkflowID string
+	Reason     string
+}
+
+// Proposal is the router's triage suggestion of candidate workflows for a chat
+// request, rendered as a choice panel with a manual-answer slot. Picking a candidate
+// launches that workflow; the custom slot continues the conversation.
+type Proposal struct {
+	ProposalID      string
+	SessionID       string
+	Prompt          string
+	OriginalRequest string
+	Candidates      []ProposeCandidate
+}
+
 // Session is the UI's view of one server session.
 type Session struct {
 	ID            string
@@ -141,6 +159,7 @@ type Session struct {
 	Events        []EventEntry
 	Pending       *Approval
 	Clar          *Clarification // open questions awaiting answers (clarification view)
+	Propose       *Proposal      // router workflow suggestion awaiting a pick (propose view)
 	Active        bool           // an inference/tool is in flight (drives the spinner)
 }
 
@@ -256,6 +275,12 @@ type Model struct {
 	clarText      []string       // per-question free-text / custom answer
 	clarTyping    bool           // typing into the custom buffer for the focused question
 	clarDismissed bool           // peeked away from the form (it can be reopened)
+
+	// workflow-propose view (the router's candidate-workflow picker)
+	proposeCursor    int    // cursor over candidates (== len(candidates) → custom slot)
+	proposeText      string // free-text answer typed into the custom slot
+	proposeTyping    bool   // typing into the custom buffer
+	proposeDismissed bool   // peeked away from the picker (it can be reopened)
 }
 
 // NewModel builds the initial idle state.
@@ -290,6 +315,10 @@ func (m Model) displayState() DisplayState {
 	if s != nil && s.Clar != nil && !m.clarDismissed {
 		return StateClarification
 	}
+	// A router workflow proposal is the operator's call to make before chatting on.
+	if s != nil && s.Propose != nil && !m.proposeDismissed {
+		return StateWorkflowPropose
+	}
 	if m.approvalDismissed {
 		return StateInSession
 	}