feat(tts): add Piper as a native local TTS provider (closes #8508) (#17885)

Piper (OHF-Voice/piper1-gpl) is a fast, local neural TTS engine from the
Home Assistant project that supports 44 languages with zero API keys.
Adds it as a native built-in provider alongside edge/neutts/kittentts,
installable via 'hermes tools' with one keystroke.

What ships:

- New 'piper' built-in provider in tools/tts_tool.py
  - Lazy import via _import_piper()
  - Module-level voice cache keyed on (model_path, use_cuda) so switching
    voices doesn't invalidate older cached voices
  - _resolve_piper_voice_path() accepts either an absolute .onnx path or a
    voice name (auto-downloaded on first use via 'python -m
    piper.download_voices --download-dir <cache>')
  - Voice cache at ~/.hermes/cache/piper-voices/ (profile-aware via
    get_hermes_dir)
  - Optional SynthesisConfig knobs: length_scale, noise_scale,
    noise_w_scale, volume, normalize_audio, use_cuda — passed through
    only when configured, so older piper-tts versions aren't broken
  - WAV output then ffmpeg conversion path (same as neutts/kittentts) so
    Telegram voice bubbles work when ffmpeg is present
  - Piper added to BUILTIN_TTS_PROVIDERS so a user's
    tts.providers.piper.command cannot shadow the native provider
    (regression test included)

- 'hermes tools' wizard entry
  - Piper appears under Voice and TTS as local free, with
    'pip install piper-tts' auto-install via post_setup handler
  - Prints voice-catalog URL and default-voice info after install

- config.yaml defaults
  - tts.piper.voice defaults to en_US-lessac-medium
  - Commented advanced knobs for discoverability

- Docs
  - New 'Piper (local, 44 languages)' section in features/tts.md
    explaining install path, voice switching, pre-downloaded voices,
    and advanced knobs
  - Piper listed in the ten-provider table and ffmpeg table
  - Custom-command-providers section updated to drop the Piper example
    (now native) and add a piper-custom example for users with their own
    trained .onnx models
  - overview.md bumps provider count to ten

- Tests (tests/tools/test_tts_piper.py, 16 tests)
  - Registration (BUILTIN_TTS_PROVIDERS, PROVIDER_MAX_TEXT_LENGTH)
  - _resolve_piper_voice_path across every branch: direct .onnx path,
    cached voice name, fresh download with correct CLI args, download
    failure, successful-exit-but-missing-files, empty voice to default
  - _generate_piper_tts: loads voice once, reuses cache, voice-name
    download wiring, advanced knobs flow through SynthesisConfig
  - text_to_speech_tool end-to-end dispatch and missing-package error
  - check_tts_requirements: piper availability toggles the return value
  - Regression guard: piper cannot be shadowed by a command provider
    with the same name
  - Pre-existing test_tts_mistral test broadened to mock the new
    piper/kittentts/command-provider checks (otherwise it false-passes
    when piper is installed in the test venv)

E2E verification (live):

Actual pip install piper-tts, config piper + en_US-lessac-low,
text_to_speech_tool call, voice auto-downloaded from HuggingFace,
WAV synthesized, ffmpeg-converted to Ogg/Opus. Second call hits the
cache (~60ms). Cache dir populated with .onnx and .onnx.json.

This caught a real bug during development: the first pass used '-d' as
the download-dir flag; the actual piper.download_voices CLI wants
'--download-dir'. Fixed before PR opened.

hermes_cli/config.py

@@ -779,7 +779,7 @@ DEFAULT_CONFIG = {
     # limit (OpenAI 4096, xAI 15000, MiniMax 10000, ElevenLabs 5k-40k model-aware,
     # Gemini 5000, Edge 5000, Mistral 4000, NeuTTS/KittenTTS 2000).
     "tts": {
-        "provider": "edge",  # "edge" (free) | "elevenlabs" (premium) | "openai" | "xai" | "minimax" | "mistral" | "neutts" (local)
+        "provider": "edge",  # "edge" (free) | "elevenlabs" (premium) | "openai" | "xai" | "minimax" | "mistral" | "gemini" | "neutts" (local) | "kittentts" (local) | "piper" (local)
         "edge": {
             "voice": "en-US-AriaNeural",
             # Popular: AriaNeural, JennyNeural, AndrewNeural, BrianNeural, SoniaNeural
@@ -809,6 +809,19 @@ DEFAULT_CONFIG = {
             "model": "neuphonic/neutts-air-q4-gguf",  # HuggingFace model repo
             "device": "cpu",  # cpu, cuda, or mps
         },
+        "piper": {
+            # Voice name (e.g. "en_US-lessac-medium") downloaded on first
+            # use, OR an absolute path to a pre-downloaded .onnx file.
+            # Full voice list: https://github.com/OHF-Voice/piper1-gpl/blob/main/docs/VOICES.md
+            "voice": "en_US-lessac-medium",
+            # "voices_dir": "",        # Override voice cache dir; default = ~/.hermes/cache/piper-voices/
+            # "use_cuda": False,       # Requires onnxruntime-gpu
+            # "length_scale": 1.0,     # 2.0 = twice as slow
+            # "noise_scale": 0.667,
+            # "noise_w_scale": 0.8,
+            # "volume": 1.0,
+            # "normalize_audio": True,
+        },
     },
     
     "stt": {