fix(transcribe): warn when optional captions are skipped

[miguel-heygen]

Summary

• emit a structured whisper_unavailable warning on stderr when transcribe --optional --json skips captions
• preserve the existing exit-0 and stdout JSON contracts for optional pipelines
• point Windows users to the official prebuilt whisper-bin-x64.zip before the CMake fallback

Regression

A Windows feedback report showed transcribe --optional exiting 0 without any stderr diagnostic when whisper-cpp was unavailable, allowing pipelines that suppress successful stdout to ship captionless output silently.

Tests

• bunx vitest run src/commands/transcribe.test.ts src/whisper/manager.test.ts
• bun run --filter @hyperframes/cli typecheck
• bunx oxlint packages/cli/src/commands/transcribe.ts packages/cli/src/commands/transcribe.test.ts packages/cli/src/whisper/manager.ts packages/cli/src/whisper/manager.test.ts
• pre-commit hooks (tracked artifacts, lint, format, fallow, typecheck)