fix(transcribe): avoid Python whisper CLI

[miguel-heygen]

What

Only discover whisper-cli when looking for a system whisper.cpp binary.

Why

The Python OpenAI Whisper package also installs a whisper executable. HyperFrames treated that unrelated CLI as whisper.cpp and passed incompatible flags, causing transcription to fail before fallback.

How

Remove the ambiguous whisper executable fallback and add a regression test that places a fake Python-style whisper on PATH.

Test plan

• bun run --filter @hyperframes/cli test (128 files, 1637 tests)
• bun run --filter @hyperframes/cli typecheck
• bunx oxfmt --check packages/cli/src/whisper/manager.ts packages/cli/src/whisper/manager.test.ts
• bunx oxlint packages/cli/src/whisper/manager.ts packages/cli/src/whisper/manager.test.ts
• git diff --check

[miguel-heygen]

Superseded by #2350, which fixes the same Whisper manager discovery bug on a current branch with focused coverage. Closing this older duplicate to keep one review path.