Rename the Shell action type to Open

[danielchalmers]

Summary

Renames the user-facing action type Shell → Open across the UI and the code, with no behavior change.

Why

"Shell" is developer jargon. What the action actually does is open something: an app, file, folder, or URL. The editor's own hint text already describes it that way ("Choose the app, file, folder, URL, or command to open"), and "Open" matches standard Windows vocabulary. This is a naming-clarity change only.

We deliberately did not add a toggle for UseShellExecute. That is a ProcessStartInfo implementation detail, not a user-meaningful choice — turning it off would break URLs, folders, and file-association launches while exposing nothing users need. If a "run a raw command line" scenario ever comes up, it belongs as its own distinct action type, not a boolean named after a .NET property.

What changed

• ActionType.Shell → ActionType.Open (enum member).
• Dropdown label and editor hint text now say Open.
• Internal identifiers renamed for consistency: CreateShellAction → CreateOpenAction, ShellActionDefaults → OpenActionDefaults (file renamed), ApplyShellDefaults/GetShellDefaults/_autoShellDefaults → Open*, ExecuteShell → ExecuteOpen, BrowseShellTarget → BrowseOpenTarget.
• Tests and AGENTS.md updated to match.
• UseShellExecute = true in the launch code is unchanged — that is the .NET API, not the feature name.

Compatibility

Settings serialize Type as an integer (Open is still 2), so existing saved configurations load unchanged. No migration needed.

Testing

• dotnet build RadialActions.sln
• dotnet test RadialActions.sln