Add regression tests for report-path collision guards

[eyeinthesky6]

Why

IndexPilot rejects overlapping input and output paths before writing a report. review has a regression test for this safety guard, but the matching guards in doctor, audit, and compare do not.

Scope

Add focused or parameterized tests for the existing guards. Do not change their runtime behavior unless a test exposes a real bug.

Done when

• doctor rejects identical JSON and Markdown output paths;
• audit rejects identical JSON and Markdown output paths;
• compare rejects any collision among its two inputs and two outputs;
• each case returns exit code 2 and checks the current error text;
• the tests prove no output is written and existing input files remain unchanged;
• python -m pytest tests/test_cli.py -q passes.

Useful files

• indexpilot/cli.py
• tests/test_cli.py

Comment here before starting so two contributors do not duplicate the work.