Document Poolside external agent setup (#62225)

# Objective

Document how to use Poolside in Zed through the ACP Registry, Poolside
Agent CLI, manual Custom Agent configuration, and Terminal Threads.

## Solution

- Add Poolside paths to the AI by Company guide.
- Document ACP Registry installation and in-thread login.
- Document CLI-assisted and manual Custom Agent configuration.
- Explain the settings-path and PATH requirements.
- Link to the public Poolside Agent CLI repository and Poolside’s Zed
documentation.

## Testing

- Ran `./script/prettier`.
- Ran `git diff --check`.
- Verified Poolside’s live ACP Registry entry.
- Verified the commands, configuration, and authentication behavior
against the Poolside and Zed implementations.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks are not applicable
- [x] The content follows Zed’s documentation and UI conventions
- [x] Code tests are not applicable to this documentation-only change
- [x] Performance impact is not applicable

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Release Notes:

- N/A
8c259313dcAmy Duquette committed on 8/5/2026, 5:34:07 PM· committed by GitHubparent65a5c89
2 files changedLine totals unavailable