Keep Slack link handling native

[dwaxe]

Slack already supports labeled links with <URL|label>, so this keeps that native format instead of adding a Markdown parser.

Bare HTTP(S) URLs are emitted as native link elements when messages are converted to rich_text blocks, such as lists and drafts. CommonMark [label](URL) input remains literal and now produces a warning with the supported Slack syntax.

This replaces #158 with a smaller implementation and no new parser or dependency.

Validation

• bun test — 404 tests passed
• tsc --noEmit
• oxlint — 0 errors
• oxfmt --check .