fix(cli): classify missing whisper-cpp as a setup gap, not a command error (#1628)

transcribe hard-failed with cli_error whenever whisper-cpp was absent. On
Linux/Docker/CI (no Homebrew, no compiler toolchain) that is unavoidable, so it
drove ~30k cli_error/day that are really "install the prerequisite" rather than
bugs — and buried genuine transcription failures in the command-error budget.

ensureWhisper now throws a typed WhisperUnavailableError when no binary exists
and none can be built. The transcribe command reports that on a dedicated
transcribe_unavailable metric instead of cli_error, and a new --optional flag
lets pipelines skip captions and exit 0. Real transcription crashes still fail
as cli_error. init and the skill pipelines already continue without captions.

Also removes a stale doc reference to a `transcribe --provider groq` flag that
does not exist.

packages/cli/src/telemetry/events.ts

@@ -330,6 +330,14 @@ export function trackCommandFailure(command: string, err: unknown): void {
   });
 }
 
+// Whisper being absent/uninstallable is an environment prerequisite gap, not a
+// command crash — track it on its own low-severity metric instead of cli_error
+// so the command-failure budget reflects real bugs. `optional` records whether
+// the caller (init / skill pipeline) treated captions as skippable.
+export function trackTranscribeUnavailable(props: { optional: boolean }): void {
+  trackEvent("transcribe_unavailable", { optional: props.optional });
+}
+
 export function trackRenderFeedback(props: {
   rating: number;
   renderDurationMs: number;