chore: untrack local .codecanvas/ artifacts

These are lean-ctx cache files (74MB callgraph.json + entrypoints.json)
that were accidentally tracked. Remove from index and gitignore.

Author: donggyun112

.codecanvas/callgraph.json

@@ -121,3 +121,6 @@ CLAUDE.md
 
 # Conformance test outputs (timestamped folders from --test mode)
 **/conformance/20*-*-*_*-*-*/
+
+# Local lean-ctx/codecanvas cache artifacts
+.codecanvas/