ci: add fmt and clippy linting workflows

- fmt.yaml: checks code formatting with cargo fmt
- clippy.yaml: runs clippy linter with warnings as errors
- both run on push and pull_request events

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

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

.tangled/workflows/clippy.yaml

···
+engine: nixery
+
+when:
+  - event: ["push", "pull_request"]
+
+dependencies:
+  nixpkgs:
+    - rustc
+    - cargo
+    - clippy
+
+steps:
+  - 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