docs(skills): add build style skill

.pi/skills/build-style

@@ -0,0 +1 @@
+../../src/skills/style/build

src/skills/style/build/SKILL.md

@@ -0,0 +1,9 @@
+---
+name: build-style
+description: Use when creating, editing, or reviewing build files, package manifests, build targets, or dependency metadata in this repository.
+---
+
+# build-style
+
+Use this skill as the source of truth for repository build conventions.
+Follow the shared style skill instructions in `src/skills/style/STYLE.md`.

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

@@ -1,4 +1,4 @@
-# Build rules
+# Rust build rules
 
 ## Use workspace dependency versions
 
@@ -14,6 +14,21 @@ Add package-local dependency features only when the feature set is specific to t
 The repository has one Cargo workspace and one `Cargo.lock`.
 Centralized dependency 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 check`, Rust Analyzer, and Bazel builds consistent.
+
 ## Declare Rust tests in Bazel
 
 ### Rule