Add CLAUDE.md with CI-monitoring instructions

CLAUDE.md

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+# Claude Code Instructions for pg_count_nulls
+
+## GitHub CI
+
+After **every** push, monitor GitHub CI to completion (`gh pr checks <pr> --watch` when the
+branch has an open PR, `gh run watch` for a branch with no PR yet) — do not consider a push
+complete until its CI run is green (or a failure is understood and explicitly accepted by
+the user).
+
+This includes the `claude-code-review` job specifically: a green job conclusion only means
+the review *ran*, not that its findings were addressed. After the job completes, fetch and
+read its actual PR comment (`gh api repos/<owner>/<repo>/issues/comments/<id>` from the
+comment URL, or `gh pr view --json comments`) for open findings.
+
+For every finding, as soon as you see it: either act on it (fix it, or explicitly state why
+it doesn't need fixing) or ask the user for direction. Never leave a finding unaddressed and
+unacknowledged just because the check itself is green.
+
+If a finding — or one remotely similar to a finding from an earlier push on the same PR —
+shows up again, STOP immediately and ask the user for help before doing anything else. A
+recurrence means you and the review bot are not in alignment (either the earlier fix didn't
+actually address it, or you and the reviewer disagree about it), and continuing to guess at
+it burns tokens without resolving the actual disagreement.
+
+Do not blindly implement review findings, either. They're generally good, but treat each one
+as a claim to verify against the actual code before acting on it — the review re-reads the
+diff each run without the code's full history or design intent in mind, and can be wrong or
+miss context.