Feature request: configurable editor CLI for shell workspace operations

## Problem

AionCore's shell module currently opens workspace folders by running `code .` (VS Code). Users who prefer other editors (Zed, Fleet, VSCodium, etc.) cannot change this without modifying the source.

## Use case

I use Zed as my daily driver. The `shell::open_workspace` flow hardcodes `code`, which means every "open in editor" action either fails silently or opens the wrong editor.

## Prior work

I had a proof-of-concept in PR #685 (now closed) that introduced an `EditorCommand` enum and shell-level dispatch (`zed .` when Zed is installed, fallback to `code .`). The approach was:

- Add a `ShellCommand::OpenEditor` variant to `crates/aionui-api-types/src/shell.rs`
- Detect installed editors at the shell service layer
- Dispatch to the right CLI

## Question

Would the project accept making the editor CLI configurable — either through auto-detection (my approach) or a user-facing setting? Happy to adapt the implementation to match the team's preferred pattern.

Start with shell::open_workspace and crates/aionui-api-types/src/shell.rs, then review the approach described from closed PR #685. Confirm with maintainers whether configuration or auto-detection is preferred before coding. Done means the accepted design lets users open workspaces with a configurable or detected editor instead of a hardcoded code command.

Written by the indexing model from the issue text.