[codex] docs: add README TOC automation

[DMontgomery40]

What changed

• add a small Python script that keeps the README table of contents synchronized inside explicit markers
• add a regression-check script for the new README note and fork freshness badges
• add GitHub Actions workflows to validate the README on PRs and keep the TOC synchronized on main

Why

• the repo is currently a single large README, so TOC drift is easy to introduce accidentally
• the regression checks protect the newly added content from being dropped in later edits
• CI makes the docs maintenance rules visible and repeatable for contributors

Stacking note

• this branch includes the README base commit from [codex] docs: clarify TSK caveat and add fork freshness badges #78 because GitHub cannot target a fork-only base branch in an upstream PR
• after [codex] docs: clarify TSK caveat and add fork freshness badges #78 merges, this PR should collapse to the automation-only diff automatically

Validation

• python3 scripts/sync_readme_toc.py --check
• python3 scripts/check_readme_regressions.py
• python3 -m compileall scripts
• git diff --check

[nelsonjchen]

I really like the TSK car warning's phrasing and would like to incorporate that, but the rest is a bit heavy and I would like to omit them. It's also kinda questionable to add a test for a doc like that but that's Codex for ya.