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/update.go

@@ -108,6 +108,9 @@ func (m Model) handleKey(k tea.KeyMsg) (tea.Model, tea.Cmd) {
 		m.clarEnsureState()
 		return m.handleClarificationKey(k)
 	}
+	if m.displayState() == StateWorkflowPropose {
+		return m.handleProposeKey(k)
+	}
 	if m.editMode == ModeInsert {
 		return m.handleInsertKey(k)
 	}