Linux package: transcription fails over MCP and the CLI (no ffmpeg executable shipped) #1221

Priority: P1. On the Linux package, an agent cannot transcribe or translate captions.

Repro

1. Install the Linux package (AppImage of the openscreen-linux CI artifact, main @ 5bc5591, run 38564622152; tested under WSLg, Ubuntu 24.04).
2. Call the MCP transcribe tool on a project, or run Openscreen captions <project.openscreen>.
3. Transcription fails.

Evidence (e2e pass of 2026-10-14, logged in #1217)

• MCP: transcribe returns {"error":"The transcription failed: stt:native-extraction-unavailable: no ffmpeg binary for native audio extraction"}.
• CLI: logs Error occurred in handler for 'stt:transcribe': FfmpegUnavailableError …, then fails with "Transcription failed: File path is not approved or is not a supported video file". The renderer-decode fallback refuses the path.
• Packaging: resources/electron/native/bin/linux-x64/ ships the ffmpeg shared libraries (also in helper-ffmpeg/) but no ffmpeg executable. A system /usr/bin/ffmpeg on PATH is not used.

Windows ships ffmpeg-shared.exe and macOS stages the SDK's ffmpeg (#616). Linux builds with fetch:ffmpeg:sdk, which skips the CLI.