cc @SomeoneToIgnore

## Summary

Follow-up to https://github.com/zed-industries/zed/discussions/55352,
where the conclusion was to split `editor.rs` incrementally by topic
instead of all at once.

This mechanically extracts two editor topics into focused sibling
modules:

- `crates/editor/src/code_actions.rs`
- `crates/editor/src/completions.rs`

One odd boundary remains: `Editor::context_menu()` is still a general
context-menu accessor, but it now lives in `code_actions.rs` because it
was part of the moved code actions block and is also used by
completions, Vim tests, agent UI, and the quick action bar. Would you
prefer that generic context-menu accessor stay in `editor.rs` for now
until context-menu code gets its own extraction?

## Testing

- `cargo check -p editor --lib`
- `cargo check -p editor --tests`
- `cargo check -p editor --lib --features test-support`

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