Replaces the inline bash markdown generation with a Python script that:
- Groups survivors by function (one section per function, function body
shown once per section, surviving mutants nested as subsections)
- Embeds each enclosing function's source via Python AST (so the agent
has full context, not just a 3-line `mutmut show` diff)
- Inlines the existing test file(s) listed in [tool.mutmut].tests_dir
- Writes an ACH-style task description at the bottom following the
prompt template from arXiv 2501.12862
Output goes to mutation-report.md (artifact) and the head of the file
is appended to $GITHUB_STEP_SUMMARY for at-a-glance visibility.
