## Implementation
- [x] Add inactivity safeguard configuration.
- [x] Add prompt service abstraction with Windows and no-op implementations.
- [x] Use native Windows app notifications with action buttons for Windows prompts.
- [x] Track last text transcript arrival and last transcript timestamp on active recording runs.
- [x] Prompt at configured inactivity thresholds and auto-stop at the configured threshold.
- [x] Use inferred end time for safeguard-triggered normal stops.
- [x] Add focused behavior tests.
- [x] Validate with `dotnet test MeetingAssistant.slnx` and `openspec validate add-recording-inactivity-safeguard --strict`.