docs(skills): add build style skill

src/skills/style/build/SKILL.md

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+---
+name: build-style
+description: Use when creating, editing, or reviewing Bazel `BUILD` files or Rust `Cargo.toml` package manifests in this repository.
+---
+
+# build-style
+
+Use this skill for Bazel `BUILD` files and Rust `Cargo.toml` package manifests.
+Follow the shared style skill instructions in `src/skills/style/STYLE.md`.

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

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+# Dependency rules
+
+## Use workspace dependency versions
+
+### Rule
+
+Declare Rust dependency versions in the repository root `Cargo.toml` under `[workspace.dependencies]`.
+Reference workspace dependencies from package `Cargo.toml` files with `workspace = true`.
+Do not declare dependency versions in package `Cargo.toml` files unless the operator explicitly requests a package-local version.
+Add package-local dependency features only when the feature set is specific to that package.
+
+### Rationale
+
+The repository has one Cargo workspace and one `Cargo.lock`.
+Centralized versions keep package manifests consistent with the workspace lockfile.
+
+## Declare Rust dependencies in each build system
+
+### Rule
+
+Declare each direct crate dependency in each build system that builds the Rust target.
+For Cargo, declare the crate in the package `Cargo.toml` file.
+Use `workspace = true` for workspace-managed dependencies.
+For Bazel, add the crate label to the `deps` of the target that uses it.
+Apply this rule to `rust_library`, `rust_binary`, and `rust_test` targets.
+
+### Rationale
+
+Cargo and Bazel resolve Rust dependencies independently.
+Complete declarations keep Cargo, Rust Analyzer, and Bazel builds consistent.

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.