Update agent docs with new capabilities

[ethanpalm]

Document the agent's improved file/image reading, PR review feedback handling, and multi-file update capabilities.

Documentation changes

Compare to #3426

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For Reviewers

When reviewing documentation PRs, please consider:

✅ Technical accuracy

• Code examples work as written
• Commands and configurations are correct
• Links resolve to the right destinations
• Prerequisites and requirements are accurate

✅ Clarity and completeness

• Instructions are clear and easy to follow
• Steps are in logical order
• Nothing important is missing
• Examples help illustrate the concepts

✅ User experience

• A new user could follow these docs successfully
• Common gotchas or edge cases are addressed
• Error messages or troubleshooting guidance is helpful

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Note

Low Risk
Documentation-only edits with no product logic changes; risk is limited to potential user confusion if any capability descriptions are inaccurate.

Overview
Updates the agent docs to reflect expanded capabilities, including multi-file search/update, reading/processing files and images (especially from Slack attachments), and using the agent to address PR review comments.

Adds new workflow guidance in agent/workflows.mdx with example prompts for handling review feedback and incorporating attached diagrams/files into documentation, and updates the Slack integration page to mention attachment support.

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