Implement config file parsing and precedence with CLI

tests/integration.rs

@@ -9,6 +9,12 @@ fn sandbox(extra_args: &[&str]) -> Command {
     cmd
 }
 
+fn write_config(dir: &TempDir, content: &str) -> String {
+    let path = dir.path().join("config.toml");
+    fs::write(&path, content).expect("failed to write config");
+    path.to_str().unwrap().to_string()
+}
+
 fn read_sid_from_stat(stat: &str) -> u32 {
     stat.trim()
         .split_whitespace()
@@ -538,3 +544,36 @@ fn rw_missing_path_errors() {
         "expected path in error message, got: {stderr}"
     );
 }
+
+#[test]
+fn config_missing_file_errors() {
+    let output = sandbox(&["--config", "/nonexistent/config.toml"])
+        .args(["--", "true"])
+        .output()
+        .expect("failed to execute");
+
+    assert!(!output.status.success());
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("/nonexistent/config.toml"),
+        "expected config path in error, got: {stderr}"
+    );
+}
+
+#[test]
+fn config_invalid_toml_errors() {
+    let dir = TempDir::new().unwrap();
+    let cfg = write_config(&dir, "not valid {{{{ toml");
+
+    let output = sandbox(&["--config", &cfg])
+        .args(["--", "true"])
+        .output()
+        .expect("failed to execute");
+
+    assert!(!output.status.success());
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("cannot parse"),
+        "expected parse error, got: {stderr}"
+    );
+}