initial: Piper TTS default plugin

Runtime monkey-patch that changes tools.tts_tool.DEFAULT_PROVIDER from
'edge' to 'piper' at plugin load time. Zero fork needed.

- plugin.yaml with requires_pip: piper-tts
- __init__.py with _apply_piper_patch() + on_session_start hook
- README with installation and safety notes

README.md

@@ -1,3 +1,57 @@
-# hermes-piper-plugin
+# Hermes Piper Plugin
 
-Hermes TTS plugin — makes Piper the default TTS provider instead of Edge. Runtime monkey-patch, zero fork needed.
+Makes Piper TTS the default provider instead of Microsoft Edge TTS.
+
+**Zero modifications to the Hermes repo.** No fork required. The plugin
+monkey-patches `tools.tts_tool.DEFAULT_PROVIDER` at plugin load time.
+
+## Why
+
+- Piper is local, free, CPU-only — no API key, no cloud dependency
+- Edge TTS requires internet access and ffmpeg for Telegram voice bubbles
+- This eliminates the need to maintain a fork just to change the default
+
+## How it works
+
+```python
+# At plugin load time, this single line runs:
+tools.tts_tool.DEFAULT_PROVIDER = "piper"
+```
+
+That's it. `DEFAULT_PROVIDER` is read at call time, so changing it after
+import is safe. The plugin also logs the effective default at session start.
+
+## Installation
+
+```bash
+# 1. Ensure piper-tts is installed
+pip install piper-tts
+
+# 2. Install the plugin from Gitea
+hermes plugins install ssh://git@code.lazyworkhorse.net:2222/Hermes/hermes-piper-plugin.git
+
+# 3. Restart gateways to load the plugin
+```
+
+## Config
+
+The plugin changes the **default** provider. You can still override per
+session by setting `tts.provider` in `~/.hermes/config.yaml`:
+
+```yaml
+tts:
+  provider: elevenlabs      # override per-session
+  voice: "Adam"             # ElevenLabs voice
+```
+
+## Requires
+
+- `piper-tts` Python package (installed via pip)
+- Piper voice models (downloaded automatically on first use, ~50MB each)
+
+## Safety
+
+- If `piper-tts` is not installed, the TTS tool returns an error asking
+  the user to install it — no silent fallback to Edge
+- Removing the plugin restores the original Hermes default (Edge)
+- No data loss risk. No changes to any file in the Hermes repo.