Add updatesite/category.xml so the eclipse-repository build is non-empty (#53)

Tycho's eclipse-repository build produced an empty p2 repo because the
module only had the legacy site.xml. With a category.xml, a plain
./mvnw clean install produces a complete per-version p2 repo at
updatesite/target/repository (feature + 8 bundles + category) with no
Eclipse GUI or p2 publisher step. This removes the README's
"remove the feature and build again" quirk from the release flow.


Claude-Session: https://claude.ai/code/session_015krtPy49YGqBERvq4yDjCa

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: khatchad

edu.cuny.citytech.refactoring.common.updatesite/category.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<site>
+  <feature id="edu.cuny.citytech.refactoring.common.feature" version="0.0.0">
+    <category name="edu.cuny.citytech.refactoring.category" />
+  </feature>
+  <category-def name="edu.cuny.citytech.refactoring.category" label="Common Refactoring Framework">
+    <description> A framework with some common functionality for refactoring tool development.
+    </description>
+  </category-def>
+</site>