Closes #63620

Count Agent Skill descriptions by Unicode characters rather than UTF-8
bytes across validation, load-time warnings, the skill creator counter,
and imported-description truncation. This aligns Zed with the Agent
Skills specification and reference validator.

Testing:

- `cargo test -p agent_skills --lib`
- `cargo test -p settings_ui --lib`
- `cargo build -p zed`
- Manually verified that a 342-character CJK description containing
1,026 UTF-8 bytes can be saved successfully
- `./script/clippy -p agent_skills -p settings_ui --tests` is blocked by
an unrelated existing `clippy::drain_collect` diagnostic in
`crates/rpc/src/proto_client.rs`

<img width="667" height="210" alt="Successfully created Unicode test
skill"
src="https://github.com/user-attachments/assets/2427fce2-9f0f-462d-959f-e08ad160b3d1"
/>

Confirmed that the Codex agent panel discovers the skill and exposes it
in command completion without a length warning.

<img width="953" height="367" alt="image"
src="https://github.com/user-attachments/assets/8c100197-2e6e-4627-b3f8-862bdfaac918"
/>

Release Notes:

- Fixed valid multibyte Agent Skill descriptions being rejected for
exceeding a byte-based length limit.

---------

Co-authored-by: Chara <249489759+cmdr-chara@users.noreply.github.com>
Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>