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/ws/client.go

@@ -82,6 +82,20 @@ func (c *Client) Send(frame []byte) {
 	}
 }
 
+// Drain returns all queued-but-unsent client frames. Test-only: meaningful before
+// Run starts consuming the send channel, so a test can assert on what was Sent.
+func (c *Client) Drain() [][]byte {
+	var out [][]byte
+	for {
+		select {
+		case f := <-c.send:
+			out = append(out, f)
+		default:
+			return out
+		}
+	}
+}
+
 // Run drives the reconnect loop until ctx is cancelled.
 func (c *Client) Run(ctx context.Context) {
 	backoff := initialBackoff