fix(utils): Inverse ignore logic yields ignored files and excludes extensionless files

[CodeMaverick-143]

Problem Description

In insight/utils.py, lines 50–52 contain a critical logic inversion:

for file in files:
    # Check for both extension and exact filename match (for Dockerfile, etc.)
    if file.lower().endswith(SUPPORTED_EXTS) or file in IGNORED_DIRS:
         yield os.path.join(root, file)

IGNORED_DIRS is populated by get_ignored_dirs() from .insightignore and default ignore sets (e.g. .env, venv). Because the condition checks:
or file in IGNORED_DIRS
Any file explicitly specified to be ignored (such as .env or files inside .insightignore) is matched and yielded as a source file to be parsed and sent to the Gemini API!

Furthermore, extensionless build/config files like Dockerfile, Makefile, Jenkinsfile are never matched because SUPPORTED_EXTS contains ".dockerfile" and ".makefile", but not the actual filenames "Dockerfile" and "Makefile".

Steps to Reproduce

1. Create a .insightignore containing .env.
2. Create a .env file with API_SECRET=supersecret and a main.py.
3. Call list_source_files(".").
4. Output includes ['.env', 'main.py']. .env is read, parsed, and its secrets are sent to Gemini!

Expected Behavior

• Files and directories in .insightignore and default ignore sets must never be yielded.
• Filenames like Dockerfile and Makefile should be matched by an explicit set of supported filenames (e.g., SUPPORTED_FILENAMES = {"dockerfile", "makefile", "containerfile", "jenkinsfile"}).

Affected Files

• insight/utils.py:45-52

Proposed Solution

1. Define SUPPORTED_FILENAMES = {"dockerfile", "makefile", "containerfile", "jenkinsfile"}.
2. Filter out ignored files:

   for file in files:
       if file in IGNORED_DIRS:
           continue
       if file.lower().endswith(SUPPORTED_EXTS) or file.lower() in SUPPORTED_FILENAMES:
           yield os.path.join(root, file)

[mikemikimike]

Implemented in PR #37.

• list_source_files now skips files listed in .insightignore, including explicit ignored-file inputs, while preserving ignored-directory traversal behavior.
• Standard extensionless source files (Dockerfile, Makefile, Containerfile, and Jenkinsfile) are now discovered case-insensitively.
• Added regression coverage for ignored files, ignored directories, explicit ignored inputs, and extensionless source filenames.

Local verification passed:

• pytest tests/: 2 passed
• Python bytecode compilation for setup.py and all insight/*.py modules
• import insight
• Ruff checks for the changed files, excluding the pre-existing UP024 finding

The GitHub Actions CI run is action_required with zero jobs for this fork pull request, so it is blocked before execution by the upstream fork-approval policy. The upstream baseline also has existing Ruff findings unrelated to this change.