fix(benchmarks): complete local setup and CI scope

[tulayha]

Summary

• initialize and verify FFmpeg/ffprobe as part of the one-command Promptfoo setup
• let VidXP install FFmpeg through a supported package manager when it is missing
• document the Homebrew prerequisite for a fresh macOS machine
• classify benchmark-only changes as container- and Desktop-neutral so they do not build product artifacts

Validation

• npm --prefix benchmarks/codex-mcp run test:setup (5 passed)
• uv run --no-sync vidxp init --yes --json
• uv run --no-sync python -m unittest tests/test_ci_scope.py -v (9 passed)
• uv run --no-sync ruff check utils/ci_scope.py tests/test_ci_scope.py
• npm exec --yes markdownlint-cli2 -- docs/benchmarking/agent_ablation.md
• git diff --check

Scope

Internal benchmark and CI tooling only. No storage or model rebuild is required.