fix: preserve reports for duplicate basenames

[mikemikimike]

Summary / Problem

Closes #24.

Reports were named only from the source basename, so files such as pkg_a/user.py and pkg_b/user.py overwrote each other and left duplicate links in summary.md.

Changes

• Add a shared report filename helper that sanitizes the normalized source path into a flat, unique report name.
• Use the same generated name when writing per-file reports and when generating summary links.
• Add a regression test proving that same-basename files retain both report contents and distinct summary links.

Tests

• .venv\\Scripts\\python.exe -m pytest --basetemp=.pytest-tmp-full tests/ -q — 1 passed.
• .venv\\Scripts\\python.exe -m py_compile setup.py insight\\__init__.py insight\\analyzer.py insight\\cli.py insight\\detector.py insight\\reporter.py insight\\utils.py — passed.
• .venv\\Scripts\\python.exe -c "import insight" — passed.
• Ruff check and format check for tests\\test_reporter.py — passed.
• Targeted Ruff check for insight\\reporter.py with the pre-existing I001 finding excluded — passed.

Compatibility / Known limitations

The report contents and summary metrics are unchanged; only report filenames and corresponding links are made path-specific. The upstream baseline currently reports existing Ruff findings, including the historical formatting/import finding in insight/reporter.py; no unrelated cleanup is included here.

Issue link

#24