feat: add video environment preflight

[jbdrak]

What Changed

Adds a dependency-free preflight command for video-use:

python helpers/check_env.py --videos-dir /path/to/your/videos

It checks:

• ffmpeg and ffprobe are available and version 4+
• the footage directory exists and is writable
• the <videos_dir>/edit/ output directory can be created and written to
• optional HyperFrames, Remotion, or Manim backends when explicitly requested with --require

It exits non-zero when a required check fails. It doesn't install packages or contact external services.

Closes #121.

Why This Approach

The repository has no runtime dependency on a specific CLI framework and no existing test suite, so the command uses only the Python standard library. Optional animation engines stay opt-in because video-use installs them lazily per animation slot.

The check writes and removes a temporary file in the target directories instead of relying only on permission bits, which reflects the real failure mode on macOS, Linux, and WSL.

Documentation

• install.md now uses the preflight command during setup verification.
• SKILL.md now recommends it on cold start and documents optional backend checks.

Verification

• python3 -m unittest discover -s tests -v: 9 passed
• python3 -m py_compile helpers/check_env.py tests/test_check_env.py: passed
• python3 -m compileall -q helpers tests: passed
• python3 helpers/timeline_view.py --help: passed
• Real temporary writable footage directory: 4 passed, 0 failed, exit 0; edit/ created
• Missing footage directory: 2 passed, 2 failed, exit 1; no directory created
• git diff --check: passed

Scope / Non-Goals

• No automatic installation
• No ElevenLabs API calls
• No transcription or rendering
• No mandatory Node, Remotion, HyperFrames, Manim, or LaTeX dependency