feat: bootstrap he maintainer skill

he-maintainer/references/rust.md

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+# Rust Harness Notes
+
+Use this reference for Rust repositories.
+
+## Common files to inspect
+
+- `Cargo.toml`
+- `Cargo.lock`
+- `src/`
+- `crates/`
+- `tests/`
+- `rust-toolchain.toml`
+
+## Common validation commands
+
+Typical examples:
+
+- `cargo test`
+- `cargo build`
+- `cargo clippy --all-targets --all-features -- -D warnings`
+- `cargo fmt --check`
+
+## Common guardrails
+
+- do not casually update `Cargo.lock`
+- check for workspace crates before describing structure
+- prefer the smallest crate-local change that solves the task
+- distinguish unit, integration, and benchmark targets when relevant