chore: add root package.xml, Init.py, and InitGui.py (#373)

Move package.xml from freecad/ to mod root following the standard
addon layout used by sdk, gears, datums, and solver.

Add root Init.py (sys.path setup for silo-client) and InitGui.py
that consolidates all Silo GUI initialization:
- SiloWorkbench class and registration
- Silo overlay context registration
- Document observer for .kc tree building
- Auth and activity dock panels via kindred_sdk
- First-start settings check
- Start page override
- kindred:// URL handler

This decouples Silo initialization from src/Mod/Create/InitGui.py,
which previously held five Silo-specific deferred setup functions.

package.xml

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<package format="1" xmlns="https://wiki.freecad.org/Package_Metadata">
+  <name>silo</name>
+  <description>PLM workbench for Kindred Create</description>
+  <version>0.1.0</version>
+  <maintainer email="development@kindred-systems.com">Kindred Systems</maintainer>
+  <license file="LICENSE">MIT</license>
+  <url type="repository">https://git.kindred-systems.com/kindred/silo-mod</url>
+
+  <content>
+    <workbench>
+      <classname>SiloWorkbench</classname>
+      <subdirectory>freecad</subdirectory>
+    </workbench>
+  </content>
+
+  <kindred>
+    <min_create_version>0.1.0</min_create_version>
+    <load_priority>60</load_priority>
+    <pure_python>true</pure_python>
+    <dependencies>
+      <dependency>sdk</dependency>
+    </dependencies>
+  </kindred>
+</package>