chore: add fix-rust-fmt Claude Code skill

.claude/skills/fix-rust-fmt/SKILL.md

@@ -0,0 +1,57 @@
+# fix-rust-fmt
+
+Fix Rust formatting issues in the optimism monorepo. When the `rust-fmt` CI job
+fails, don't bother reading the errors — just run the formatter and commit
+whatever it changes.
+
+## When to Use
+
+Use this skill when the `rust-fmt` CI job fails on a PR that touches Rust code.
+
+### Trigger Phrases
+
+- "Fix rust formatting"
+- "rust-fmt is failing"
+- "Fix the rust-fmt CI job"
+
+## Prerequisites
+
+- `mise` must be trusted and installed for this repo (`mise trust && mise install`)
+
+## Workflow
+
+### Step 1: Ensure mise tools are installed
+
+From the repo root (or worktree root):
+
+```bash
+cd <REPO_ROOT> && mise install
+```
+
+This installs `rust`, `just`, and all other tools pinned in `mise.toml`.
+
+### Step 2: Install the nightly toolchain with rustfmt
+
+The justfile pins a specific nightly (see `NIGHTLY` variable in `rust/justfile`).
+Install it:
+
+```bash
+cd <REPO_ROOT>/rust && mise exec -- just install-nightly
+```
+
+### Step 3: Run the formatter
+
+```bash
+cd <REPO_ROOT>/rust && mise exec -- just fmt-fix
+```
+
+Any files that change are the reason the CI job failed. Stage and commit them.
+
+## Notes
+
+- `mise exec --` activates the mise environment so `cargo`, `just`, and
+  `rustup` resolve to the versions pinned in `mise.toml`.
+- The nightly toolchain is required because the workspace uses unstable
+  `rustfmt` options (see `rust/rustfmt.toml`).
+- There is no need to inspect `rust-fmt` CI errors — if the job failed, running
+  `just fmt-fix` and committing the result is the complete fix.