fix(desktop): prefer bundled sidecars in releases

[fitz2882]

Summary

• prefer binaries beside the desktop executable in installed release builds
• preserve target-directory priority for debug development builds
• deduplicate search directories while retaining their priority order
• cover release and debug ordering with focused unit tests

Why

An installed release could discover a source checkout's target binary before the sidecars bundled with the app. That makes the installed application depend on unrelated local build artifacts and can mix versions after an update.

Related issue

None found. Existing open sidecar issues concern different packaging, signing, or platform-specific failures.

Testing

• focused release/debug search-order unit tests
• strict desktop Clippy
• Rust formatting and desktop file-size checks
• just ci

Manual testing

1. Install a release build that includes bundled sidecars while a source checkout contains separately built sidecar binaries.
2. Start a managed agent from the installed application.
3. Verify from the application logs that the bundled sidecar beside the desktop executable was selected.
4. In a debug build, verify that the target-directory binary still takes priority.