feat(workspace): add embedded MPV to the command palette

Register an "Switch player to Embedded MPV" command in the Cmd+K command
palette so the embedded MPV player can be activated like the other players.
Visibility is gated on an async getEmbeddedMpvSupport() check, mirroring the
Settings dropdown so the command only appears when embedded MPV is usable.

Generalizes the per-command visibility flag from desktopOnly to a `requires`
discriminator ('none' | 'managed-external' | 'embedded-mpv').

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/architecture/workspace-shell.md

@@ -184,11 +184,16 @@ Command palette behavior is shell-owned but view-extensible:
    visibility — a navigation command like `Open sources` is invisible while
    the user is on `/workspace/sources` but the id stays in storage so it
    reappears in the recent section after navigating away.
-7. Five "Switch player to X" commands are registered globally by
-   `WorkspacePlayerCommandsContributor`. The MPV/VLC entries are visible only
-   in Electron, and the entry matching the current `SettingsStore.player()`
-   value is disabled. The new player setting applies to the next playback
-   session; an existing stream is not re-mounted.
+7. Six "Switch player to X" commands are registered globally by
+   `WorkspacePlayerCommandsContributor` (VideoJS, HTML5, ArtPlayer, Embedded
+   MPV, MPV, VLC). Each command carries a `requires` flag gating its
+   visibility: the MPV/VLC ("managed-external") entries are visible only when
+   `RuntimeCapabilitiesService.supportsManagedExternalPlayers` is true, and the
+   Embedded MPV ("embedded-mpv") entry is visible only after an async
+   `window.electron.getEmbeddedMpvSupport()` check resolves to `supported`
+   (mirroring the Settings dropdown gate). The entry matching the current
+   `SettingsStore.player()` value is disabled. The new player setting applies
+   to the next playback session; an existing stream is not re-mounted.
 
 Keyboard shortcut help is shell-owned: