Fix pre-existing clippy warnings in arc-workflows

- Replace useless format!() with .to_string() in parse_decision
- Derive Default for HookDecision instead of manual impl
- Use contains_key() instead of get().is_none() in semantic parser

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-workflows/src/hook/executor.rs

@@ -96,7 +96,7 @@ impl HookExecutorImpl {
                 return decision;
             }
             HookDecision::Block {
-                reason: Some(format!("hook exited with code 2")),
+                reason: Some("hook exited with code 2".to_string()),
             }
         } else {
             HookDecision::Block {

crates/arc-workflows/src/hook/types.rs

@@ -108,9 +108,10 @@ pub struct PromptHookResponse {
 }
 
 /// Decision returned by blocking hooks.
-#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+#[derive(Debug, Clone, Default, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(tag = "decision", rename_all = "snake_case")]
 pub enum HookDecision {
+    #[default]
     Proceed,
     Skip {
         #[serde(default)]
@@ -125,11 +126,6 @@ pub enum HookDecision {
     },
 }
 
-impl Default for HookDecision {
-    fn default() -> Self {
-        Self::Proceed
-    }
-}
 
 impl HookDecision {
     /// Merge two decisions. Block > Skip/Override > Proceed.

crates/arc-workflows/src/parser/semantic.rs

@@ -109,7 +109,7 @@ impl SemanticState {
             Self::add_class_to_node(node, cls);
         }
         // Legacy: translate codergen_mode to type if type is not explicitly set
-        if node.attrs.get("type").is_none() {
+        if !node.attrs.contains_key("type") {
             if let Some(mode) = node.attrs.get("codergen_mode").and_then(AttrValue::as_str) {
                 let mapped = match mode {
                     "one_shot" => "prompt",