feat(stt): add free local whisper transcription via faster-whisper

Replace OpenAI-only STT with a dual-provider system mirroring the TTS
architecture (Edge TTS free / ElevenLabs paid):

  STT: faster-whisper local (free, default) / OpenAI Whisper API (paid)

Changes:
- tools/transcription_tools.py: Full rewrite with provider dispatch,
  config loading, local faster-whisper backend, and OpenAI API backend.
  Auto-downloads model (~150MB for 'base') on first voice message.
  Singleton model instance reused across calls.
- pyproject.toml: Add faster-whisper>=1.0.0 as core dependency
- hermes_cli/config.py: Expand stt config to match TTS pattern with
  provider selection and per-provider model settings
- agent/context_compressor.py: Fix .strip() crash when LLM returns
  non-string content (dict from llama.cpp, None). Fixes #1100 partially.
- tests/: 23 new tests for STT providers + 2 for compressor fix
- docs/: Updated Voice & TTS page with STT provider table, model sizes,
  config examples, and fallback behavior

Fallback behavior:
- Local not installed → OpenAI API (if key set)
- OpenAI key not set → local whisper (if installed)
- Neither → graceful error message to user

Co-authored-by: Jah-yee <Jah-yee@users.noreply.github.com>

hermes_cli/config.py

@@ -194,8 +194,13 @@ DEFAULT_CONFIG = {
     },
     
     "stt": {
-        "enabled": True,
-        "model": "whisper-1",
+        "provider": "local",  # "local" (free, faster-whisper) | "openai" (Whisper API)
+        "local": {
+            "model": "base",  # tiny, base, small, medium, large-v3
+        },
+        "openai": {
+            "model": "whisper-1",  # whisper-1, gpt-4o-mini-transcribe, gpt-4o-transcribe
+        },
     },
     
     "human_delay": {