Automation skill references are not highlighted after reopening

Testing #334386

## Reproduction

1. Open the Automations view and create an Automation.
2. In the prompt editor, type `/` and accept two valid Agent Host skill suggestions.
3. Verify both accepted skills receive inline reference styling.
4. Save the Automation.
5. Reopen the Automation from the Automations list.

## Actual

The prompt text is preserved, but both valid skill references are shown as plain text after reopening. Waiting several seconds and refocusing the prompt does not restore the styling.

Typing another skill prefix still shows valid skill suggestions. Accepting a new completion causes all previously saved valid skill references to suddenly regain their inline reference styling.

## Expected

Valid saved skill references should be revalidated against the active Agent Host and highlighted as soon as the Automation is reopened, without requiring another completion to be accepted.

## Build

- VS Code Insiders 1.137.0-insider
- Commit `de8cc55dae905582f191fdcfb6dff8c811a743c4`
- macOS arm64

## Screenshot