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/config.py

@@ -23,6 +23,7 @@ class Config:
         "exa_search": ["exa_api_key"],
         "twitter_xreach": ["twitter_auth_token", "twitter_ct0"],  # legacy key name; used by twitter-cli
         "groq_whisper": ["groq_api_key"],
+        "openai_whisper": ["openai_api_key"],
         "github_token": ["github_token"],
     }