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.

pyproject.toml

@@ -0,0 +1,18 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "hermes-piper-plugin"
+version = "1.0.0"
+description = "Piper TTS plugin for Hermes Agent"
+authors = [{name = "@gortium"}]
+license = {text = "MIT"}
+requires-python = ">=3.11"
+dependencies = []
+
+[project.entry-points.'hermes_agent.plugins']
+piper = "hermes_piper_plugin:register"
+
+[tool.setuptools.packages.find]
+include = ["hermes_piper_plugin*", "tests*"]