agent-box-transcribe: no backend on NixOS boxes (no distro python3, no whisper-cli) #972

Follow-up to #970 / PR #971. agent-box-transcribe runs the prebuilt pywhispercpp manylinux wheel under the distro python3 (/usr/bin/python3). A NixOS box has no distro python, so unless whisper-cli is on PATH the helper exits with "no prebuilt whisper engine for this host".

Options, none decided:

1. Build whisper-cpp in agent-box's own CI and push it to a binary cache the boxes trust (the same producer gap as Bake the nixpkgs git cache into the VM image: first harness install 36s → 11s, zero per-user disk #669: nixpkgs#whisper-cpp is not on cache.nixos.org at the pinned rev, and building it on a small box runs it out of memory).
2. Run the manylinux wheel under a nix python plus nix-ld or an FHS wrapper so libstdc++/libc resolve. Fragile; needs a check on a real NixOS box.
3. Ship a static whisper.cpp binary from a pinned release of ours.

Recommendation: option 1, since it also lets the Nix path replace the wheel on distro boxes. Not run on a NixOS box yet, so (2) is untested.