Fixes #55436

The create-directory agent tool was classified as a read tool, which
caused
  the agent UI to render it with the search icon.

This PR marks `create_directory` as an edit tool and gives it a
folder-add
icon in the conversation tool card. Creating a directory mutates the
project,
so treating it as an edit tool better matches the behavior and avoids
the
  misleading search icon.

  Validation:
  - `cargo build -p zed`
- `cargo test -p agent test_create_directory_tool_kind_is_edit --
--nocapture`
  - `git diff --check`
- Manually verified in a local dev build that asking the agent to create
`tmp-
zed-icon-test-dir` shows a folder icon instead of a search icon, and
that the
  directory is created successfully.

  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:

  - Fixed create-directory agent tool cards showing a search icon.

<img width="593" height="281" alt="07547c9f-2cf2-432b-8c0f-86f336dce3c5"
src="https://github.com/user-attachments/assets/a5a277ee-56c5-4801-a7d8-27c347222169"
/>
<img width="594" height="157" alt="2b3f2b62-4782-42f1-a20d-c26f9cfe8940"
src="https://github.com/user-attachments/assets/37845785-5145-4701-9060-1dd1bdb60801"
/>

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>