Support Telegram as a channel for CORE agent

[AggManik]

Add Telegram as a first-class messaging channel for CORE agent.

Scope

• send + receive messages
• auth flow (bot token / setup UX)
• webhooks/update handling + retries
• message formatting parity with WhatsApp/Slack (text, mentions, attachments where possible)
• reminders delivery via Telegram
• channel selection: user can choose Telegram as the active channel (alongside WhatsApp/Slack)

Notes

• keep channel abstractions consistent with existing WhatsApp/Slack implementations

[harshithmullapudi]

#736