fix(cli): reject whitespace-only prompts in embed to match rerank

## Summary

`mlxcel embed -p " "` is accepted while `mlxcel rerank -d " "` is rejected. The two commands' otherwise line-for-line identical input validation diverges on whitespace-only strings. Bring embed in line with rerank.

## Background

Both `run_embed` and `run_rerank` guard against empty inputs early, with matching "nothing to embed/rerank" and "image file does not exist" wording. The empty check differs: embed tests `String::is_empty`, so a whitespace-only prompt passes, while rerank tests `d.trim().is_empty()`, so it is rejected.

## Proposed Solution

Adopt the `trim().is_empty()` check in `embed.rs` so whitespace-only prompts fail with the same style of error as rerank. Add one unit test following the `src/commands/*_tests.rs` convention. Also rename embed's misleading binding: it is named `empty` but holds an index, producing `bail!("prompt {empty} is empty")`.

## Implementation Notes

- `src/commands/embed.rs:135` uses `args.prompts.iter().position(String::is_empty)` and binds the index as `empty`.
- `src/commands/rerank.rs:108` uses `args.documents.iter().position(|d| d.trim().is_empty())` and binds it as `index`.
- The `src/commands/` directory already has a `_tests.rs` convention to follow.

## Acceptance Criteria

- [ ] `mlxcel embed` rejects whitespace-only prompts with the same style of error as `rerank`
- [ ] A test pins the behavior

---

## Original Suggestion

### Title: fix(cli): embed accepts whitespace-only prompts while rerank rejects them

`mlxcel embed -p " "` is accepted while `mlxcel rerank -d " "` is rejected: the two commands' otherwise line-for-line identical input validation differs on whitespace-only strings.

## Evidence

- `src/commands/embed.rs:135-137` — checks `.position(String::is_empty)` (whitespace passes)
- `src/commands/rerank.rs:108-110` — checks `.position(|d| d.trim().is_empty())` (whitespace rejected)
- The surrounding `run_*` prologues are otherwise identical (same "nothing to embed/rerank" and "image file does not exist" wording)

Minor bonus in the same lines: embed's binding is named `empty` but holds an index (`bail!("prompt {empty} is empty")`), which reads oddly.

## Suggested fix

Adopt the `trim().is_empty()` check in `embed.rs` (and rename the index binding), plus one unit test; `src/commands/` already has a `_tests.rs` convention to follow.

## Acceptance criteria

- [ ] `mlxcel embed` rejects whitespace-only prompts with the same style of error as `rerank`
- [ ] A test pins the behavior

Start with the validation at src/commands/embed.rs:135 and compare it with src/commands/rerank.rs:108. Follow the src/commands/*_tests.rs convention to add a unit test for whitespace-only prompts, and verify that mlxcel embed rejects them with the expected error style and that the index binding is clear.

Written by the indexing model from the issue text.