Triggers: scheduled, webhook, and new Slack Channel trigger improvements

⚡ Triggers just got a lot smarter
You now have more control over when your agents' triggers run, what they see, and how they stop. Every triggered-run conversation also has a Stop button and a chat box, so you can steer or troubleshoot a run without starting over.

🗓️ Scheduled Triggers
• 🧵 Conversation mode: keep one ongoing conversation across runs instead of starting fresh each time. Each run builds on the last.
• ⏹️ Run limits: set an end date, a maximum number of runs, or "run once." When a trigger reaches its limit, it moves to Ended.
• ⏸️ Auto-pause: a trigger can pause itself after a set number of failures in a row, and you get an email when it does.

🔗 Webhook Triggers
• 🎯 Payload filters: run only when the payload matches your conditions (up to 10, using equals, contains, exists, and more). Anything that doesn't match is skipped.
• 🧩 Payload templating: drop payload values straight into your instructions with {{path}}, or include the whole payload.
• 🧵 Conversation mode: continue one conversation across runs, or keep one conversation per key, such as one per ticket ID. Duplicate deliveries won't start duplicate runs.
• 🔐 Signed webhooks: require callers to sign their requests (Standard Webhooks, GitLab, or Slack). Unsigned, invalid, or replayed requests are refused. This is opt-in, and existing webhooks don't change.
• ⏸️ Reliable pause: pause and enable now work every time.

✨ New Trigger Type: Slack Channel Trigger
• 💬 Run on new messages: an agent runs automatically when a new top-level message is posted in a Slack channel it watches.
• 🎛️ You choose what fires it: any message, specific keywords, or an advanced pattern. You also choose whether only bots and integrations can fire it, or everyone in the channel.
• 👀 Preview first: see which recent messages would have fired before you turn it on. Great for monitoring alert channels.
• ℹ️ Note: this option only appears for agents that already have a Slack connector connected.

📚 More detailed documentation for each of these updates will be available soon in ReadMe.