fix: resolve clippy warnings in config and llm modules

Derive Default instead of manual impl for RawConfig and CliConfig,
and use strip_prefix instead of manual slicing in LLM parser.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Alan Shum

src/config.rs

@@ -18,7 +18,7 @@ pub enum AiCli {
 }
 
 /// Raw JSON-serializable config (matches the file format).
-#[derive(Debug, Deserialize)]
+#[derive(Debug, Default, Deserialize)]
 #[serde(default)]
 struct RawConfig {
     #[serde(rename = "preferred-ai-cli")]
@@ -27,27 +27,12 @@ struct RawConfig {
     copilot: CliConfig,
 }
 
-#[derive(Debug, Deserialize)]
+#[derive(Debug, Default, Deserialize)]
 #[serde(default)]
 struct CliConfig {
     model: Option<String>,
 }
 
-impl Default for RawConfig {
-    fn default() -> Self {
-        Self {
-            preferred_ai_cli: None,
-            claude: CliConfig::default(),
-            copilot: CliConfig::default(),
-        }
-    }
-}
-
-impl Default for CliConfig {
-    fn default() -> Self {
-        Self { model: None }
-    }
-}
 
 /// Model tier for intelligent cross-backend mapping.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]

src/grouper/llm.rs

@@ -63,8 +63,7 @@ pub async fn request_grouping(
         .lines()
         .filter_map(|line| {
             let line = line.trim();
-            if line.starts_with("FILE: ") {
-                let rest = &line[6..];
+            if let Some(rest) = line.strip_prefix("FILE: ") {
                 let end = rest.find(" (")?;
                 Some(&rest[..end])
             } else {