28pins add alerts

[Sam Clark (28pins)]

Summary

What changed: Added support for alerts, like [!NOTE]
Why: to give users and LLMs more flexibility and better UX

API change: BlockQuoteType now includes BlockQuoteAlertKind as an optional. See below:

Old:

text(String)
case nested([BlockQuoteType])

New:

case text(String, BlockQuoteAlertKind?)
case nested([BlockQuoteType], BlockQuoteAlertKind?)

Validation

Checked demos and stress-tested kitchen sink demo

OSS readiness

• No secrets, internal URLs, private identifiers, or product-only service names were added.
• Public docs, fixtures, or notices were updated if behavior or dependencies changed.
• Third-party dependency changes (adds, removes, version bumps) are intentional and reviewed.
• Streaming/incomplete markdown behavior remains covered by fixtures or tests.

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