feat(transcribe): Whisper transcription module with Groq→OpenAI fallback (#277)

Adds agent_reach/transcribe.py (download → compress → chunk → transcribe with provider fallback, fully mocked tests). Maintainer follow-up on the branch: wired an agent-reach transcribe CLI subcommand + skill docs so agents can actually invoke it, added the missing configure openai-key branch the error hint referenced, removed two dead static methods. 103 tests pass; wheel-gate clean.

agent_reach/skill/references/video.md

@@ -39,6 +39,17 @@ yt-dlp --dump-json "ytsearch5:query"
 > **字幕注意**: 手动上传的字幕提取可靠；自动生成字幕可能存在行间重复，需后处理。
 > **评论注意**: `--write-comments` 基于网页抓取（非 YouTube Data API），部分评论可能丢失。
 
+### 无字幕兜底：Whisper 音频转写
+
+```bash
+# 视频没有字幕时的兜底：下载音频并用 Whisper 转写（Groq 免费 key 即可）
+agent-reach transcribe "https://www.youtube.com/watch?v=VIDEO_ID"
+agent-reach transcribe ./local_audio.mp3 -o /tmp/transcript.txt
+```
+
+> 需要先配置 key：`agent-reach configure groq-key gsk_xxx`（免费，console.groq.com）
+> 或 `agent-reach configure openai-key sk-xxx`。默认 auto 模式：groq 失败自动降级 openai。
+
 ## B站 / Bilibili (yt-dlp + bili-cli)
 
 ### 视频元数据 (yt-dlp)