feat: auto-onboarding when a channel is registered

After wiring a channel to an agent group, register.ts writes a task
message to the session that triggers the /welcome container skill.
The agent introduces itself immediately — the user sees typing and
then a greeting without having to send a message first.

Uses kind 'task' (not 'system') so the poll loop picks it up normally.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

container/skills/welcome/SKILL.md

@@ -0,0 +1,25 @@
+---
+name: welcome
+description: Introduce yourself to a newly connected channel. Triggered automatically when a channel is first wired. Send a friendly greeting and brief overview of what you can do.
+---
+
+# /welcome — Channel Onboarding
+
+You've just been connected to a new messaging channel. Introduce yourself to the user.
+
+## What to do
+
+1. Send a short, friendly greeting using `send_message`
+2. Mention your name (from your CLAUDE.md)
+3. Briefly describe 2-3 things you can help with based on your configured skills and tools
+4. Keep it to 2-4 sentences — don't overwhelm
+
+## Tone
+
+Warm but concise. This is a first impression — be helpful, not verbose. Match the channel's vibe (casual for Telegram/Discord, slightly more professional for Slack/Teams/email).
+
+## Example
+
+> Hey! I'm Andy, your assistant. I can help with coding tasks, answer questions, manage scheduled reminders, and work with files. Just send me a message anytime.
+
+Adapt based on your actual name and capabilities. Don't list every tool — pick the most useful ones.