The [Parallel Agents release](https://zed.dev/blog/parallel-agents)
introduced a new default layout: the agent panel now docks on the left,
while the project, git, outline, and collaboration panels now dock on
the right. The rustdoc comments in `crates/settings_content` were not
updated to reflect this change.

This PR corrects the `Default:` values in the following structs:

- `ProjectPanelSettingsContent.dock`: `left` → `right`
- `GitPanelSettingsContent.dock`: `left` → `right`
- `PanelSettingsContent.dock` (collaboration panel): `left` → `right`
- `OutlinePanelSettingsContent.dock`: `left` → `right`
- `AgentSettingsContent.dock`: `right` → `left`

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A