Ignore event-model's and client-core's target/, like server's own

The event-model commit picked up its build directory because there was
no gitignore entry for it -- server/target/ is listed explicitly rather
than a blanket target/, and the new crates need the same line each.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

.gitignore

@@ -7,6 +7,8 @@ local.properties
 .idea/
 .DS_Store
 server/target/
+event-model/target/
+client-core/target/
 
 # Server logs from a development run (ai-server.log by convention,
 # wg-test.log from ./test-wg-tunnel.sh).