Convert to pip package

- Moved __init__.py and plugin.yaml into hermes_piper_plugin/ package directory
- Added pyproject.toml with build config and entry point registration
- Updated tests to reference new subdirectory paths
- Updated README with pip-based installation instructions

This converts the plugin from a flat layout (loaded via hermes plugins install)
to a proper pip package with the 'hermes_agent.plugins' entry point.

hermes_piper_plugin/plugin.yaml

@@ -0,0 +1,11 @@
+name: piper
+version: 1.0.0
+description: >
+  Makes Piper TTS the default provider instead of Edge. Zero fork needed —
+  uses runtime monkey-patching to set DEFAULT_PROVIDER = "piper" at plugin
+  load time. Survives Hermes upgrades without merge conflicts.
+author: "@gortium"
+hooks:
+  - on_session_start
+requires_pip:
+  - piper-tts