fix: respect ignored files during source discovery

[mikemikimike]

Summary / Problem

Closes #21.

list_source_files incorrectly yielded files listed in .insightignore, including sensitive files such as .env, so those files could be read and sent to the Gemini API. It also failed to discover standard extensionless source files such as Dockerfile and Makefile.

Changes

• Skip ignored files for recursive discovery and explicit file inputs.
• Keep ignored directories out of traversal.
• Recognize Dockerfile, Makefile, Containerfile, and Jenkinsfile case-insensitively.
• Add regression tests covering configured ignores, default ignored directories, explicit ignored files, and extensionless source filenames.

Tests

• .venv\\Scripts\\python.exe -m pytest --basetemp=.pytest-tmp-final tests/ -q — 2 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.
• .venv\\Scripts\\ruff.exe check insight\\utils.py tests\\test_utils.py --ignore UP024 — passed.
• .venv\\Scripts\\ruff.exe format --check tests\\test_utils.py — passed.

Compatibility / Known limitations

The change preserves the existing extension matching behavior and adds only explicit filename matching for common extensionless source files. The upstream baseline currently reports 13 existing Ruff findings outside this change, including the pre-existing UP024 finding in insight/utils.py; no unrelated cleanup is included here.

Issue link

#21