RD-623: Argmax OSS engine and pipelines (replace legacy WhisperKit transcription)

[EduardoPach]

What does this PR do?

This change integrates the Argmax open-source Swift CLI (argmax-cli from argmaxinc/argmax-oss-swift) into OpenBench as a first-class engine and pipeline set, and retires the old Python WhisperKit transcription module that wrapped a different code path.

Engine

• Adds ArgmaxOpenSourceEngine (argmax_oss_engine.py): resolves the CLI via optional cli_path, or clone + swift build -c release --product argmax-cli under ARGMAX_OSS_CACHE_DIR (default ~/.cache/openbench/argmax-oss), with optional commit_hash pin.
• Exposes transcribe and diarize helpers that shell out to argmax-cli with caller-supplied flag lists.

Pipelines

• Transcription: ArgmaxOpenSourceTranscriptionPipeline — runs transcribe, parses JSON report segments/words (including per-word timings when present).
• Diarization: ArgmaxOpenSourceDiarizationPipeline — runs diarize, produces RTTM compatible with existing annotation loading.
• Orchestration: ArgmaxOpenSourceOrchestrationPipeline — diarize then transcribe and merge speaker labels using word timing.

Aliases and removals

• Legacy whisperkit.py transcription pipeline is removed.
• Existing whisperkit-* transcription aliases (e.g. tiny, large-v3, large-v3-turbo) now target the OSS transcription pipeline and configs (same alias names for benchmark continuity).
• New aliases: argmax-oss-diarization, argmax-oss-orchestration-tiny (plus orchestration config pattern consistent with other orchestration entries).

Testing

• Exercise openbench-cli evaluate (or pipeline smoke) with a whisperkit-* or argmax-oss-* alias after ensuring argmax-cli is built or cli_path is set.