planning: add issue refactor-statusline-error-prefix to 2.1

refactor statusline error prefix from class name to CAT

Author: cowwoc

.cat/issues/v2/v2.1/refactor-statusline-error-prefix/index.json

@@ -0,0 +1 @@
+{"status": "open"}

.cat/issues/v2/v2.1/refactor-statusline-error-prefix/plan.md

@@ -0,0 +1,17 @@
+# Plan
+
+## Goal
+
+Update statusline error prefix from class name to 'CAT' — change error format from '⚠ <class name>: <error message>' to '⚠ CAT: <error message>'
+
+## Pre-conditions
+
+(none)
+
+## Post-conditions
+
+- [ ] All statusline error strings in StatuslineCommand.java use the `⚠ CAT:` prefix instead of class name prefixes (StatuslineCommand, MalformedJson, etc.)
+- [ ] Test assertions updated to verify the exact `⚠ CAT:` prefix, not just the warning symbol
+- [ ] Javadoc in StatuslineCommand.getActiveIssue() updated to reflect `⚠ CAT: <message>` format
+- [ ] All tests passing with no regressions
+- [ ] E2E verification: statusline errors display `⚠ CAT: <error message>` instead of `⚠ <class name>: <error message>`