Initial commit: AYON Matrix Publish addon

- package.py: addon manifest (v0.1.0)
- server/__init__.py: BaseServerAddon definition
- server/ayon_matrix_publish/plugins/publish/integrate_matrix.py:
  Pyblish plugin that uploads published representations to Matrix
  rooms during the Integration stage. Supports:
  - Upload to Matrix content repository (mxc:// URI)
  - Formatted messages with preview images
  - Auto room creation per asset
  - Configurable via Studio Settings or env vars
- create_package.py: builds distributable .zip package

server/__init__.py

@@ -0,0 +1,15 @@
+"""Matrix Publish addon — server-side definition."""
+
+from ayon_server.addons import BaseServerAddon
+
+from .version import __version__
+
+
+class MatrixPublishAddon(BaseServerAddon):
+    name = "matrix_publish"
+    title = "Matrix Publish"
+    version = __version__
+
+    async def setup(self):
+        """Initialize addon — no persistent services needed."""
+        pass