feat(assistant): fileops, the Do row's local operations

One CLI for converting, extracting, merging and compressing staged files,
answering with one JSON line. Results are written under a temporary name
and renamed to the first free one, so a failure or a stop leaves nothing
half written, and a stop kills the tool's whole process group. This
ffmpeg has no libx264 and no AAC encoder, so MP4 is H.264 through VAAPI
with MP3 audio, and audio extraction copies AAC, MP3 and Opus tracks as
they are. The test builds its inputs and checks every result's type.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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