feat(telegram): render markdown messages

[birdmanmandbir]

Summary

• Render outbound Telegram text through telegramify-markdown-go.
• Send Telegram message entities instead of raw Markdown markers.
• Keep 4096-character splitting on rendered Telegram chunks.

Verification

• go test ./internal/telegram
• make test
• make ci currently fails on existing lint issues outside this change: goconst findings across multiple packages and unused internal/gitprovider/provider.go:tailString.