readme_toc.py misparses tilde and longer Markdown fences

Summary

The README TOC generator only recognizes backtick fences and treats every line beginning with three backticks as a close. It therefore includes headings from tilde fences and from content inside a longer backtick fence.

Reproduction

Run the README TOC generator on:

~~~markdown

Before

~~~markdown

Not a heading

~~~

After

~~~

Also try a four-backtick fence containing a literal triple-backtick line.

Expected: headings inside either fenced block are excluded, and the longer fence remains open until a closing fence with the same marker and at least the same length.

Actual: headings inside tilde fences are treated as headings, and a triple-backtick content line can prematurely close a longer backtick fence.

Root cause

The parser used a boolean toggled by line.strip().startswith("`"); it did not track fence marker type or length.

PR-ready reference

I prepared a tested reference branch containing a minimal parser fix and regression tests:

• Commit: https://github.com/Hughhhhcoder/codex/commit/57516831cf353aab3a73c4e99136ab987a00d1db
• Branch: codex/codex-readme-fence

The patch tracks the fence character and opening length, ignores backtick openers whose info string contains a backtick, and accepts only valid matching closers.

Validation

python -m pytest -c /tmp/codex-empty.ini --noconftest -q scripts/test_readme_toc.py — 3 passed.

Because the current contribution policy does not accept external code contributions or pull requests, I am submitting this analysis and reference commit here instead of opening a PR. Please feel free to implement or adapt the fix.