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.

website/docs/user-guide/features/overview.md

@@ -31,7 +31,7 @@ Hermes Agent includes a rich set of capabilities that extend far beyond basic ch
 - **[Browser Automation](browser.md)** — Full browser automation with multiple backends: Browserbase cloud, Browser Use cloud, local Chrome via CDP, or local Chromium. Navigate websites, fill forms, and extract information.
 - **[Vision & Image Paste](vision.md)** — Multimodal vision support. Paste images from your clipboard into the CLI and ask the agent to analyze, describe, or work with them using any vision-capable model.
 - **[Image Generation](image-generation.md)** — Generate images from text prompts using FAL.ai. Nine models supported (FLUX 2 Klein/Pro, GPT-Image 1.5/2, Nano Banana Pro, Ideogram V3, Recraft V4 Pro, Qwen, Z-Image Turbo); pick one via `hermes tools`.
-- **[Voice & TTS](tts.md)** — Text-to-speech output and voice message transcription across all messaging platforms, with nine provider options: Edge TTS (free), ElevenLabs, OpenAI TTS, MiniMax, Mistral Voxtral, Google Gemini, xAI, NeuTTS, and KittenTTS.
+- **[Voice & TTS](tts.md)** — Text-to-speech output and voice message transcription across all messaging platforms, with ten native provider options: Edge TTS (free), ElevenLabs, OpenAI TTS, MiniMax, Mistral Voxtral, Google Gemini, xAI, NeuTTS, KittenTTS, and Piper — plus custom command providers for any local TTS CLI.
 
 ## Integrations