ci: consolidate fmt and clippy into check workflow

following tangled.org/core pattern where linting and testing are
combined in one workflow with multiple steps

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

.tangled/workflows/clippy.yaml .tangled/workflows/check.yaml

···
   nixpkgs:
     - rustc
     - cargo
+    - rustfmt
     - clippy
 
 steps:
+  - name: check formatting
+    command: |
+      cargo fmt --check
+
   - name: run clippy
     command: |
       cargo clippy -- -D warnings

.tangled/workflows/fmt.yaml

···
-engine: nixery
-
-when:
-  - event: ["push", "pull_request"]
-
-dependencies:
-  nixpkgs:
-    - rustc
-    - cargo
-    - rustfmt
-
-steps:
-  - name: check formatting
-    command: |
-      cargo fmt --check