feat(skills): add rust style skill

src/skills/rust-style/RULE-FORMAT.md

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+# Rule format
+
+Use this structure for each rule:
+
+```md
+## Rule title
+
+### Rule
+
+State enforceable instructions.
+
+### Rationale
+
+Explain why the rule exists.
+```
+
+Add `### Examples` only when examples clarify the rule.

src/skills/rust-style/SKILL.md

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+---
+name: rust-style
+description: Use when creating, editing, or reviewing Rust files in this repository.
+---
+
+# rust-style
+
+Use this skill as the source of truth for repository Rust conventions.
+
+## Load rules
+
+Read every Markdown file directly under [`rules/`](rules/) before applying this skill.
+Resolve [`rules/`](rules/) relative to this `SKILL.md`.
+
+Finish when all discovered rule files have been read.
+
+## Apply rules
+
+Apply the loaded rules when editing or reviewing Rust files.
+When loaded rules conflict, follow the most specific rule.
+
+After editing Rust files, run:
+
+```bash
+nix run .#fix
+```
+
+Finish when `nix run .#fix` succeeds and the Rust change follows the loaded rules.
+If the command fails, report the failure.
+
+## Maintain rules
+
+When adding or changing files under [`rules/`](rules/), read [`RULE-FORMAT.md`](RULE-FORMAT.md) first.
+Finish when the changed rule files follow that format.

src/skills/rust-style/rules/bazel.md

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+# Bazel rules
+
+## 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.

src/skills/rust-style/rules/tests.md

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+# Test rules
+
+## Keep Rust tests in separate files
+
+### Rule
+
+Put Rust tests in separate `*_test.rs` files.
+Do not add inline `#[cfg(test)]` modules.
+Declare each test file in the nearest Bazel `BUILD` file with a `rust_test` target.
+
+### Rationale
+
+Separate test files keep production modules focused and match the repository's Bazel test layout.