Add CLAUDE.md pointing at ai/ repo's consolidated conventions

[jnasbyupgrade]

This repo had no `CLAUDE.md` yet. Adds one referencing
Postgres-Extensions/ai's `CLAUDE.md`, `PR.md`, and `CODE_STYLE.md` for
the cross-repo conventions (CI monitoring, PR/commit conventions, comment
style, etc.) rather than starting a fresh, eventually-duplicative copy
here.

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