Expand error status test coverage

- Added explicit status validation across error handling tests.
- Added lifecycle and slot-leak checks to older tests.
- Improved unhandled-error propagation coverage.
- Added repository guidance and a test runner script.
- Verified all 23 CTest tests pass.

Co-Authored by Codex GPT 5.4

AGENTS.md

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+# Repository Guidelines
+
+## Project Structure & Module Organization
+
+This is a small C library built with CMake. Core implementation lives in
+`src/error.c`. The public header is generated at build time from
+`include/akerror.tmpl.h` by `scripts/generrno.sh`, which also generates
+`src/errno.c` under the build directory. CMake package and pkg-config templates
+are in `cmake/` and `akerror.pc.in`. Tests are one-file C programs in `tests/`;
+shared test helpers live beside them, such as `tests/err_capture.h`.
+
+## Build, Test, and Development Commands
+
+Use an out-of-tree build:
+
+```sh
+cmake -S . -B build
+cmake --build build
+ctest --test-dir build --output-on-failure
+```
+
+`cmake -S . -B build` configures the build and generates `akerror.h`.
+`cmake --build build` compiles `libakerror` and test executables. `ctest`
+runs the registered unit tests. To emit CI-style results, use:
+
+```sh
+ctest --test-dir build --output-on-failure --output-junit "$(pwd)/ctest-junit.xml"
+```
+
+Mutation testing is available through:
+
+```sh
+cmake --build build --target mutation
+scripts/mutation_test.py --target src/error.c --threshold 65
+```
+
+## Coding Style & Naming Conventions
+
+Use C99-compatible C and follow the surrounding style. Functions and types use
+the `akerr_` prefix; macros and constants use `AKERR_` or all-caps macro names
+such as `PREPARE_ERROR`. Keep generated-code changes in templates or generator
+scripts, not in build outputs. Preserve concise comments for invariants,
+macro constraints, and non-obvious error lifecycle behavior.
+
+## Testing Guidelines
+
+Add tests as `tests/err_<behavior>.c`. Register each new test in the
+`AKERR_TESTS` list in `CMakeLists.txt`; tests that intentionally abort must
+also be listed in `AKERR_WILL_FAIL_TESTS`. Prefer focused executable tests that
+return zero on success and use existing helpers such as `AKERR_CHECK`. Run the
+CTest suite before submitting changes, and run mutation testing when changing
+core control-flow, reference counting, stack-trace, or handler behavior.
+
+## Commit & Pull Request Guidelines
+
+Recent commits use short, imperative, sentence-case subjects, for example
+`Fix refcount leak and stack-trace buffer overflow`. Keep commits focused and
+describe the observable behavior changed. Pull requests should include a brief
+summary, tests run, and any compatibility impact for public macros, generated
+headers, installation paths, or CMake/pkg-config consumers.
+
+## Agent-Specific Instructions
+
+Do not overwrite uncommitted user changes. Avoid editing generated files in
+`build/`; update `include/akerror.tmpl.h`, `src/error.c`, CMake files, tests,
+or scripts instead.