Filter Linux workspace open targets by XDG category

[wy3z]

Summary

• restrict generic Linux desktop-entry discovery to file managers, text editors, and terminal emulators
• use the standard XDG Categories values FileManager, TextEditor, and TerminalEmulator
• keep unrelated desktop applications out of BB's Open in/Open with menus

Root cause

PR #403 added generic Linux desktop-application discovery for workspace open
targets. It currently includes every visible .desktop entry with
Type=Application, an Exec command, and neither Hidden=true nor
NoDisplay=true.

As a result, BB's Open in/Open with menus can contain nearly every installed
desktop application on Linux, including media players, settings utilities,
messaging clients, and other software that is not a useful workspace target.

This change additionally requires a relevant standard XDG category:
FileManager, TextEditor, or TerminalEmulator. Existing explicit BB launch
adapters remain unaffected.

Validation

• pnpm exec turbo run typecheck test --filter=@bb/local-open-targets --force
• git diff --check

AGENT GENERATED