planning: add 2.1-fix-preprocessor-error-stacktrace

Author: cowwoc

.cat/issues/v2/v2.1/fix-preprocessor-error-stacktrace/index.json

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+{
+  "status": "open"
+}

.cat/issues/v2/v2.1/fix-preprocessor-error-stacktrace/plan.md

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+# Plan
+
+## Goal
+
+Fix preprocessor error to show full stacktrace instead of first line only. When a preprocessor directive fails, the error message currently shows only the exception class and method (e.g., `io.github.cowwoc.cat.hooks.skills.GetOutput.<init>()`). It should show the complete stack trace including all frames and cause chains to aid debugging.
+
+## Pre-conditions
+
+(none)
+
+## Post-conditions
+
+- [ ] Bug fixed: preprocessor errors display the full stack trace, including all frames and cause chains
+- [ ] Regression test added: test verifies full stacktrace is included in the error output
+- [ ] No new issues introduced
+- [ ] E2E verification: reproduce the preprocessor error scenario and confirm the full stacktrace appears in the output