Expose media input simulation (camera roll, photo/video, mic)

There is no camera roll injection, photo/video capture simulation, or microphone input. Apps with camera flows, profile photo upload, or voice features are hard to test. iOS supports `simctl addmedia`; Android supports pushing files into media directories and simulating camera input.

**Proposal:** Add commands or documented flows to:
- **Photos:** Add one or more images (and optionally videos) to the device's photo library / gallery, so the app can pick them (e.g. "choose from library").
- **Camera:** Simulate camera capture (e.g. provide a static image or short video as "camera" input) where the platform allows.
- **Microphone:** Simulate or inject audio input for voice features where the platform allows.

Start with photo/library injection (highest impact); document platform limits (e.g. simulator vs device).

**Acceptance criteria:**
- [ ] Add image (and optionally video) to device photo library/gallery for session device
- [ ] Document or implement camera simulation if feasible
- [ ] Document or implement microphone simulation if feasible
- [ ] Docs and skill updated