Add compatibility_flags to Slack agent configuration

[thomasgauvin]

The compatibility flag is needed because mime-types uses require('path')

Summary

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Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/agents/	@irvinebroque, @rita3ko, @elithrar, @thomasgauvin, @threepointone, @cloudflare/pcx-technical-writing

[github-actions]

Preview URL: https://59c47f2b.preview.developers.cloudflare.com
Preview Branch URL: https://thomasgauvin-patch-11.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/agents/guides/slack-agent/	https://thomasgauvin-patch-11.preview.developers.cloudflare.com/agents/guides/slack-agent/