chore(ci): add Copilot review instructions

.github/copilot-instructions.md

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+# Copilot review instructions
+
+## Reviewer focus
+
+- Review **changes in this PR**, not the whole repo. The diff is the assignment.
+- Skip files whose first ~3 lines contain `// Ported from <upstream>`,
+  `// Generated`, or `// Auto-generated`. Their contract is the upstream's,
+  not ours; surface a one-line note instead of line-level findings.
+- Skip files in `node_modules/`, `dist/`, `build/`, `bin/`, `obj/`, generated
+  Roslyn artifacts (`*.g.cs`), and lockfiles (`package-lock.json`,
+  `pnpm-lock.yaml`, `yarn.lock`).
+- Skip vendored fixtures under `**/fixtures/**` and `**/test-data/**` —
+  they're deliberately broken or deliberately verbatim.
+
+## Coordinate with other reviewers
+
+- CodeRabbit and Claude Code Review run on the same PR. **Don't repeat
+  findings they've already raised** — read the existing review comments
+  before posting.
+- If CodeRabbit has labeled a finding `false_positive` or the human author
+  has marked it resolved, don't re-raise it.
+
+## Style
+
+- Group findings by file, not by severity, when there are >5.
+- Don't suggest renames of public exports without a clear caller-side
+  benefit. The cost of a rename is paid by every consumer.
+- Don't suggest adding tests "for completeness" — only when the changed
+  contract is uncovered by existing tests.
+
+## Project conventions to respect
+
+- Node code: ESM, Node ≥20, no external runtime deps unless already
+  declared in `package.json`.
+- .NET code: nullable enabled, central package management
+  (`Directory.Packages.props`), `Version.props` is the single owner of
+  versions — never edit `<Version>` lines directly.
+- Don't suggest patterns that contradict `CLAUDE.md`, `AGENTS.md`, or the
+  repo's `.coderabbit.yaml` `path_instructions`.
+
+## Rate-limit / failure behavior
+
+If you hit a rate limit, **surface the limit and the unblock date in your
+review body** rather than the generic "encountered an error" string. The
+human author needs the date to plan, not a vague retry hint.