fix(cron): add one-time reminders and fix data paths to workspace

- Add at_seconds parameter for one-time reminders (e.g., "remind me in 10 minutes")
- Update every_seconds description to emphasize recurring-only usage
- Route cron delivery: deliver=true sends directly, deliver=false uses agent
- Fix cron data path from ~/.picoclaw/cron to workspace/cron
- Fix sessions path from workspace/../sessions to workspace/sessions

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/channels/base.go

@@ -61,7 +61,7 @@ func (c *BaseChannel) HandleMessage(senderID, chatID, content string, media []st
 		return
 	}
 
-	// 生成 SessionKey: channel:chatID
+	// Build session key: channel:chatID
 	sessionKey := fmt.Sprintf("%s:%s", c.name, chatID)
 
 	msg := bus.InboundMessage{
@@ -70,8 +70,9 @@ func (c *BaseChannel) HandleMessage(senderID, chatID, content string, media []st
 		ChatID:     chatID,
 		Content:    content,
 		Media:      media,
-		Metadata:   metadata,
 		SessionKey: sessionKey,
+		Metadata:   metadata,
+	}
 	}
 
 	c.bus.PublishInbound(msg)