Clarify how agent.flexible affects agent.default_width (#63020)

## Objective

`agent.default_width` is described as the Agent Panel’s default width,
but panels docked to the left or right use proportional sizing while
`agent.flexible` is enabled. Because flexible sizing is enabled by
default, double-clicking the outer resize handle restores the default
proportion rather than the configured pixel width. This can make
`agent.default_width` appear to be ignored.

## Solution

Clarify that `agent.default_width` controls the fixed width used when
`agent.flexible` is disabled, while resetting a flexibly sized Agent
Panel restores its default proportion.

Update the default settings comments, settings documentation, Settings
UI description, and visual customization guide. Add `agent.flexible` to
the Agent Panel configuration example and explain how it affects reset
behavior. No sizing behavior changes.

## Testing

- `cargo fmt --all -- --check`
- No runtime testing—the change only updates settings descriptions and
documentation.

## 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 adheres to Zed’s UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

assets/settings/default.json

@@ -1146,13 +1146,16 @@
     "button": true,
     // Where to dock the agent panel. Can be 'left', 'right' or 'bottom'.
     "dock": "left",
-    // Whether the agent panel should use flexible (proportional) sizing.
+    // Whether the agent panel should use flexible (proportional) sizing when docked
+    // to the left or right. When enabled, `default_width` does not control the panel
+    // width, and resetting the panel restores the default proportion.
     //
     // Default: true
     "flexible": true,
     // Where to position the threads sidebar. Can be 'left' or 'right'.
     "sidebar_side": "left",
-    // Default width when the agent panel is docked to the left or right.
+    // Default fixed width when the agent panel is docked to the left or right and
+    // `flexible` is false.
     "default_width": 640,
     // Default height when the agent panel is docked to the bottom.
     "default_height": 320,