feat(skills): add use-local-whisper skill package (#702)

Thanks for the great contribution @glifocat! This is a really well-structured skill — clean package, thorough docs, and solid test coverage. Hope to see more skills like this from you!

.claude/skills/use-local-whisper/modify/src/transcription.ts.intent.md

@@ -0,0 +1,39 @@
+# Intent: src/transcription.ts modifications
+
+## What changed
+Replaced the OpenAI Whisper API backend with local whisper.cpp CLI execution. Audio is converted from ogg/opus to 16kHz mono WAV via ffmpeg, then transcribed locally using whisper-cpp. No API key or network required.
+
+## Key sections
+
+### Imports
+- Removed: `readEnvFile` from `./env.js` (no API key needed)
+- Added: `execFile` from `child_process`, `fs`, `os`, `path`, `promisify` from `util`
+
+### Configuration
+- Removed: `TranscriptionConfig` interface and `DEFAULT_CONFIG` (no model/enabled/fallback config)
+- Added: `WHISPER_BIN` constant (env `WHISPER_BIN` or `'whisper-cli'`)
+- Added: `WHISPER_MODEL` constant (env `WHISPER_MODEL` or `data/models/ggml-base.bin`)
+- Added: `FALLBACK_MESSAGE` constant
+
+### transcribeWithWhisperCpp (replaces transcribeWithOpenAI)
+- Writes audio buffer to temp .ogg file
+- Converts to 16kHz mono WAV via ffmpeg
+- Runs whisper-cpp CLI with `--no-timestamps -nt` flags
+- Cleans up temp files in finally block
+- Returns trimmed stdout or null on error
+
+### transcribeAudioMessage
+- Same signature: `(msg: WAMessage, sock: WASocket) => Promise<string | null>`
+- Same download logic via `downloadMediaMessage`
+- Calls `transcribeWithWhisperCpp` instead of `transcribeWithOpenAI`
+- Same fallback behavior on error/null
+
+### isVoiceMessage
+- Unchanged: `msg.message?.audioMessage?.ptt === true`
+
+## Invariants (must-keep)
+- `transcribeAudioMessage` export signature unchanged
+- `isVoiceMessage` export unchanged
+- Fallback message strings unchanged: `[Voice Message - transcription unavailable]`
+- downloadMediaMessage call pattern unchanged
+- Error logging pattern unchanged