feat: add pytest tests for piper plugin

Add 15 test cases covering manifest validation, monkey-patch logic,
idempotency, error handling, lifecycle hooks, and plugin registration.

Update README with test instructions and corrected install URL.

README.md

@@ -27,12 +27,29 @@ import is safe. The plugin also logs the effective default at session start.
 # 1. Ensure piper-tts is installed
 pip install piper-tts
 
-# 2. Install the plugin from Gitea
+# 2. Install the plugin from the internal Hermes repo
 hermes plugins install ssh://git@code.lazyworkhorse.net:2222/Hermes/hermes-piper-plugin.git
 
 # 3. Restart gateways to load the plugin
 ```
 
+## Testing
+
+```bash
+cd /opt/data/projects/hermes-piper-plugin
+pip install pytest pyyaml
+python -m pytest tests/ -v
+```
+
+Tests cover:
+
+- **Manifest validation** — confirms plugin.yaml structure and required fields
+- **Monkey-patch logic** — verifies DEFAULT_PROVIDER is changed to 'piper'
+- **Idempotency** — calling `_apply_piper_patch()` twice is safe
+- **Error handling** — import failures and exceptions are caught gracefully
+- **Lifecycle hooks** — on_session_start fires without errors
+- **Plugin registration** — register() wires tools, hooks, and logging correctly
+
 ## Config
 
 The plugin changes the **default** provider. You can still override per