feat: add STT voice transcription for all messaging channels (#28)

* feat: add STT voice transcription for all channels

Automatically transcribes audio/voice messages (Telegram, WeChat, Slack,
etc.) into text before the agent sees them. Enabled via config, off by default.

Changes:
- EvoScientist/stt.py: new STT engine using faster-whisper with lazy
  model loading and per-language model selection (zh/en/auto)
- EvoScientist/channels/base.py: hook in _enqueue_raw() to transcribe
  audio files and prepend transcript to message text; removes the raw
  [voice: ...] annotation after successful transcription so the agent
  does not attempt further audio processing
- EvoScientist/config/settings.py: stt_enabled (default False),
  stt_language (default "auto")
- pyproject.toml: optional [stt] dependency group (faster-whisper>=1.0)
- tests/test_stt.py: unit tests covering all backends and channel integration

Usage:
  pip install 'EvoScientist[stt]'
  EvoSci config set stt_enabled true
  EvoSci config set stt_language zh   # zh / en / auto

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: remove unused imports (ruff F401)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: address PR #28 reviewer feedback

Changes per SemiGlassFace review (CHANGES_REQUESTED):

1. Cache config at channel __init__ — no longer calls load_config() on
   every incoming message; STT settings stored as instance attributes
   (_stt_enabled, _stt_language, _stt_model, _stt_device,
   _stt_compute_type) set once during Channel.__init__().

2. Replace deprecated asyncio.get_event_loop() with get_running_loop()
   to avoid DeprecationWarning on Python 3.12+.

3. Annotation removal now uses exact path matching instead of substring
   search — checks fp == a or a.endswith(f": {fp}]") so only the
   correct annotation is removed after transcription.

4. Expose stt_model, stt_device, stt_compute_type as config fields so
   users can override the HuggingFace model id, inference device, and
   quantisation without touching code. transcribe_file() forwards all
   three to the engine.

Also: _engines dict replaced with single _engine + _engine_key tuple
(model_id, device, compute_type) — reuses cached model unless settings
change, simpler than a dict.

Tests: 19 STT-specific tests all pass; total 1105 tests green, ruff clean.

* fix: resolve ruff lint errors (UP037, I001, PT006)

* style: apply ruff format

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -63,6 +63,7 @@ discord = ["discord.py>=2.3"]
 slack = ["slack-sdk>=3.27", "aiohttp>=3.9"]
 wechat = ["pycryptodome>=3.20"]
 qq = ["qq-botpy>=1.0"]
+stt = ["faster-whisper>=1.0"]
 oauth = ["ccproxy-api>=0.2.4"]
 all-channels = [
     "python-telegram-bot>=21.0",