docs(skills): add build style skill

src/skills/style/build/rules/tests.md

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+# Test rules
+
+## Declare Rust tests in Bazel
+
+### Rule
+
+Declare Rust test files in the nearest Bazel `BUILD` file with `rust_test`.
+Do not add `[[test]]` entries to `Cargo.toml` unless the operator explicitly requests Cargo test declarations.
+
+### Rationale
+
+This repository declares Rust test targets in Bazel.
+Keeping test declarations in Bazel avoids duplicate test target lists.
+
+## Use one source input style per `rust_test`
+
+### Rule
+
+Do not set both `crate` and `srcs` on a `rust_test` target.
+Use exactly one input style for each Rust test target.
+
+### Rationale
+
+`rules_rust` rejects `rust_test` targets that set both `crate` and `srcs`.
+Keeping one input style per target makes the test declaration valid and unambiguous.