Improve README heading parsing and Python virtualenv detection

[ahmetmizrap1232-sys]

Two small hygiene gaps are still reproducible on the current codebase:

• README heading detection can treat headings inside fenced code blocks as real documentation sections. For example, a Python snippet containing # Installation and # Usage can satisfy the README checks even when those sections do not exist in the document.
• Python virtual environment directories such as .venv/, venv/, and env/ are common local artifacts, but they are not currently reported by the junk file scan when left in the repository.

Expected behavior:

• Markdown headings inside fenced code blocks should be ignored by README section checks.
• Common local Python virtual environment directories should be reported as junk unless they are ignored by .gitignore.

Note: CI-oriented exit codes are already implemented: RepoDx returns 0 when no issues are found, 1 when hygiene issues are found, and 2 for an invalid path.

[omerbek]

The two behavior changes requested here are now covered on main:

• Ignore README headings inside code blocks #9 ignores heading-like lines inside fenced README code blocks.
• Report unignored Python virtualenv folders #11 reports .venv/, venv/, and env/ as junk unless they are ignored.

Local validation after merging both changes:

Ran 26 tests in 0.070s
OK

Closing this as resolved. The README exit-code documentation idea from #10 is useful but separate polish, not required to close this behavior issue.