[setrf]

Why this PR

Screenshot-only agents currently install and import MoviePy through stirrup.core.models. MoviePy requires Pillow below 12, so it prevents installing browser-use 0.13.11+ (which requires Pillow 12.3), even when the agent never uses audio or video.

Approach

Move MoviePy into stirrup[media], retain it in stirrup[all], and import it only inside audio/video validation and conversion methods. Missing media support raises an actionable installation error. Existing audio/video users must install the media extra; text and image use stay unchanged.

Add a CI matrix for core and media installations, covering image use without MoviePy, missing-extra errors, and real audio/video transcoding. Update installation documentation.

Validation

• 373 tests passed, 34 browser/Docker/E2B tests deselected before adding the transcoding regression test.
• Core installation: optional-media test passed, media test skipped, verified MoviePy absent.
• Media installation: both optional-media tests passed with real WAV/MP4 inputs.
• Full Ruff formatting, lint, and ty checks passed with all extras installed.
• Dependent eval installed browser-use 0.13.14 successfully; native checks and one Luna trial passed.

The lockfile also contains uv's platform-marker normalization; the seven-day release-age floor is unchanged.