docs: document Slack attachment support for shared/forwarded messages

[inkeep]

Summary

Documents the new Slack attachment handling feature added in #2317, which allows agents to receive shared and forwarded message content as context.

Changes

• Added a new "Shared and forwarded messages" subsection to the Slack Usage Guide
• Explains that agents receive the full content of shared/forwarded messages along with metadata (author, channel)
• Maintains consistent style with existing documentation

Related PR

Closes documentation gap for #2317 (fix(slack-work-apps): add attachment content to slack messages)

[vercel]

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Project	Deployment	Actions	Updated (UTC)
agents-api	Ready	Preview, Comment	Feb 24, 2026 8:08pm
agents-docs	Ready	Preview, Comment	Feb 24, 2026 8:08pm
agents-manage-ui	Ready	Preview, Comment	Feb 24, 2026 8:08pm

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