docs: add repository overview

[sit-d]

Summary

• Add a top-level README.md so visitors can quickly find the repository's Erdős #962, OEIS, Project Euler, and standalone fasmg/math explorations.
• Document that there is no repository-wide build command and point to the per-area Python/assembler entry points that currently exist.
• Fix a malformed four-backtick closing fence in erdos/962/readme.md so the source-link code block renders correctly.

Verification

• python3 - <<'PY' ... PY checked local Markdown links in README.md and erdos/962/readme.md; all local links resolved.
• python3 -m py_compile erdos/962/plot_kn_bounds.py erdos/962/verify.py erdos/962/verify_chain.py
• git diff --cached --check

Notes / Limitations

• I did not run the full plotting workflow because it requires installing matplotlib/mpmath; this PR only changes documentation and a Markdown fence.
• I did not compile the assembly/fasmg examples from the Linux cron environment because the repository does not define a single cross-platform build command.

[gemini-code-assist]

Code Review

This pull request introduces a repository-wide README.md file that outlines the contents, tooling notes, and licensing of the math repository. Additionally, it fixes a minor syntax typo in erdos/962/readme.md by correcting a code block closing delimiter. There are no review comments, so no feedback is provided.

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