fix(semgrep): extract CWE from list-form metadata (was always null)

semgrep emits metadata.cwe as a LIST (e.g. "CWE-798: Use of Hard-coded
Credentials"), but the extractor called v.as_str() on it -> always None, so SAST
findings never carried a CWE at all (silently breaking control mapping AND the
CWE-based dedup). Handle list + bare-string forms and normalise to the CWE-NNN
id. Found by the live control-triage end-to-end test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

compliance-agent/src/pipeline/semgrep.rs

@@ -82,10 +82,7 @@ impl Scanner for SemgrepScanner {
                 finding.file_path = Some(r.path);
                 finding.line_number = Some(r.start.line);
                 finding.code_snippet = Some(r.extra.lines);
-                finding.cwe = r
+                finding.cwe = r.extra.metadata.as_ref().and_then(extract_cwe);
-                    .extra
-                    .metadata
-                    .and_then(|m| m.get("cwe").and_then(|v| v.as_str()).map(|s| s.to_string()));
                 finding
             })
             .collect();
@@ -124,10 +121,34 @@ struct SemgrepExtra {
     metadata: Option<serde_json::Value>,
 }
 
+/// semgrep emits `metadata.cwe` as a list of strings like
+/// `"CWE-798: Use of Hard-coded Credentials"` (occasionally a bare string). Take
+/// the first entry and normalise it to just the `CWE-NNN` id.
+fn extract_cwe(metadata: &serde_json::Value) -> Option<String> {
+    let raw = metadata.get("cwe")?;
+    let text = match raw {
+        serde_json::Value::Array(items) => items.first()?.as_str()?,
+        serde_json::Value::String(s) => s.as_str(),
+        _ => return None,
+    };
+    let id = text.split(':').next().unwrap_or(text).trim();
+    (!id.is_empty()).then(|| id.to_string())
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
+    #[test]
+    fn extract_cwe_handles_list_and_normalises() {
+        let md = serde_json::json!({"cwe": ["CWE-798: Use of Hard-coded Credentials"]});
+        assert_eq!(extract_cwe(&md).as_deref(), Some("CWE-798"));
+        let bare = serde_json::json!({"cwe": "CWE-89"});
+        assert_eq!(extract_cwe(&bare).as_deref(), Some("CWE-89"));
+        let none = serde_json::json!({"severity": "ERROR"});
+        assert_eq!(extract_cwe(&none), None);
+    }
+
     #[test]
     fn deserialize_semgrep_output() {
         let json = r#"{