chore: add consistent mise.toml, justfile, and tooling config

justfile

@@ -13,3 +13,18 @@ run *args: _ensure_tools
 # Run with verbose logging
 verbose *args: _ensure_tools
     @uv run -m donglora_mux --verbose {{args}}
+
+# Run all checks (fmt, lint)
+check: fmt-check lint
+
+# Format code
+fmt: _ensure_tools
+    @uv run ruff format .
+
+# Check formatting without changing files
+fmt-check: _ensure_tools
+    @uv run ruff format --check .
+
+# Lint
+lint: _ensure_tools
+    @uv run ruff check .