feat: add per-route message dispatch modes (inject/parallel/queue)

Introduce three inbound message handling modes for channel adapters:

- inject (default, /btw): when a route has an active agent stream,
  inject the new user message into the running stream via the SDK's
  PrepareStep hook between tool rounds. The message is interleaved at
  the correct position in the persisted round.
- parallel (/now): start a new agent stream immediately, running
  concurrently with any existing stream (preserves current behavior).
- queue (/next): enqueue the message and process it after the current
  stream completes.

Key components:
- RouteDispatcher: per-route state management with inject channel,
  task queue, and active-stream tracking.
- PrepareStep integration: drains inject channel between tool rounds,
  records insertion position via InjectedRecorder for correct
  persistence ordering.
- interleaveInjectedMessages: inserts injected user messages at their
  actual injection position within the persisted message round.
- Parallel mode isolation: /now streams do not interact with the
  dispatcher, preventing them from clearing another stream's active
  state.

internal/agent/agent.go

@@ -98,6 +98,47 @@ func (a *Agent) runStream(ctx context.Context, cfg RunConfig, ch chan<- StreamEv
 	if readMediaState != nil {
 		prepareStep = readMediaState.prepareStep
 	}
+
+	initialMsgCount := len(cfg.Messages)
+
+	if cfg.InjectCh != nil {
+		basePrepare := prepareStep
+		prepareStep = func(p *sdk.GenerateParams) *sdk.GenerateParams {
+			if basePrepare != nil {
+				if override := basePrepare(p); override != nil {
+					p = override
+				}
+			}
+			for {
+				select {
+				case injected, ok := <-cfg.InjectCh:
+					if !ok {
+						break
+					}
+					text := strings.TrimSpace(injected.HeaderifiedText)
+					if text == "" {
+						text = strings.TrimSpace(injected.Text)
+					}
+					if text != "" {
+						insertAfter := len(p.Messages) - initialMsgCount
+						p.Messages = append(p.Messages, sdk.UserMessage(text))
+						if cfg.InjectedRecorder != nil {
+							cfg.InjectedRecorder(text, insertAfter)
+						}
+						a.logger.Info("injected user message into agent stream",
+							slog.String("bot_id", cfg.Identity.BotID),
+							slog.Int("insert_after", insertAfter),
+						)
+					}
+					continue
+				default:
+				}
+				break
+			}
+			return p
+		}
+	}
+
 	opts := a.buildGenerateOptions(cfg, tools, prepareStep)
 
 	streamResult, err := a.client.StreamText(ctx, opts...)