Add `description` field to actions

When Cockpit starts and a BlueOS extension offers new actions, the "BlueOS Extension Features" modal lists them by name only. With names like "Trigger camera at 192.168.2.10" and "Trigger all cameras" side by side, it's difficult to tell what each one actually does or which one to pick, and since the modal re-opens on every launch until every item is accepted or ignored, the user is asked to decide something the dialog never explains. Extensions have no way to help: a description on an action in the manifest would help.

Root cause: ActionConfigSchema in src/libs/blueos/schemas.ts (lines 83-89) declares only id, name, type, config and version. Zod objects strip unknown keys by default, so ExtrasJsonSchema.parse(rawData) in src/libs/blueos.ts:77 drops description from every action before getActionsFromBlueOS hands the configs to the UI. Parsing a sample manifest through the shipped schema keeps id, name, type, config and yields action.description === undefined, while the joystick group and suggestion descriptions survive — the two joystick schemas in the same file already declare description: z.string().optional() (lines 99 and 110), which is exactly what the action schema is missing.

The UI side is already built and merely starved: src/components/ExternalFeaturesDiscoveryModal.vue renders getActionDescription(action) on each new-action card (lines 53-55), so the field would display as soon as it survives parsing. That render path is unreachable today, and two things keep the compiler quiet about it — ActionConfig (src/types/cockpit-actions.ts, lines 127-149) has no description member, and getActionDescription reads it through an (action as any).description cast (lines 1224-1227), so yarn typecheck never flags the mismatch.