feat: make Telegram adapter optional

[roger704]

Summary

Makes the Telegram adapter optional, allowing deployments that only use GitHub webhooks.

Changes:

• Remove TELEGRAM_BOT_TOKEN from required environment variables
• Make Telegram adapter initialization conditional (mirroring GitHub pattern)
• Validate at least one platform adapter is configured
• Update startup messages to reflect which adapters are active
• Handle graceful shutdown when Telegram is disabled

Motivation

When using this project for GitHub-only deployments (via webhooks), the current code requires TELEGRAM_BOT_TOKEN to be set, causing startup failures. This change allows:

# GitHub-only deployment
GITHUB_TOKEN=xxx WEBHOOK_SECRET=xxx npm start

# Telegram-only deployment  
TELEGRAM_BOT_TOKEN=xxx npm start

# Both platforms
GITHUB_TOKEN=xxx WEBHOOK_SECRET=xxx TELEGRAM_BOT_TOKEN=xxx npm start

Test plan

• Verify app starts with only GITHUB_TOKEN + WEBHOOK_SECRET
• Verify app starts with only TELEGRAM_BOT_TOKEN
• Verify app fails gracefully when no adapters configured
• Verify graceful shutdown works with Telegram disabled

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