download_audio: delegate the policy to yb.download.download_audio_bounded #122

yb now ships the link-to-audio policy as one function (reeleehq/yb#44, PR reeleehq/yb#48, yb >= 0.1.24 once released): target check, playlist/live/duration refusal, pinned caps, mp3, copyright notice, per-call result directory.

Make download_audio a thin caller: keep creds.check() and as_tool_error; pass creds.opts as extra_opts (restricted to yb.download.DEPLOYMENT_OPT_KEYS, else ValueError), _docs._validate_target as validate_target, the env-tunable caps as max_bytes/max_duration_s; ingest result.path then remove result.path.parent; carry result.notice. Bump the yb[download] floor. reelee and muvid can then add thin ingest tools the same way (see reeleehq/reelee#280).