ci: add Copilot code review instructions with scope rules

Author: polaz

.github/instructions/code-review.instructions.md

@@ -0,0 +1,26 @@
+---
+applyTo: "**/*.rs"
+---
+
+# Code Review Instructions for fjall (structured-world fork)
+
+## Scope Rules (CRITICAL)
+
+- **Review ONLY code within the PR's diff.** Do not suggest inline fixes for unchanged lines.
+- For issues in code **outside the diff**, suggest creating a **separate issue** instead of proposing code changes. Example: "Consider opening an issue to validate buffer size here — this is outside this PR's scope."
+- **Read the PR description carefully.** If the PR body has an "out of scope" section listing items handled by other PRs, do NOT flag those items.
+- This fork has **multiple feature branches in parallel**. A fix that seems missing in one PR may already exist in another open PR. Check the "out of scope" section for cross-references.
+
+## Rust Standards
+
+- `unsafe` blocks require `// SAFETY:` comments explaining the invariant
+- Prefer `#[expect(lint)]` over `#[allow(lint)]` — `#[expect]` warns when suppression becomes unnecessary
+- Use `TryFrom`/`TryInto` for fallible conversions; `as` casts need `#[expect(clippy::cast_possible_truncation)]` with reason
+- No `unwrap()` / `expect()` on I/O paths — use `Result` propagation
+- `expect()` is acceptable for programmer invariants (lock poisoning) with `#[expect(clippy::expect_used, reason = "...")]`
+- Code must pass `cargo clippy --all-features -- -D warnings`
+
+## Testing
+
+- No mocks for storage — use real on-disk files via `tempfile::tempdir()`
+- Test naming: `fn <what>_<condition>_<expected>()`