feat: add ecc2 decision log audit trail

ecc2/src/session/mod.rs

@@ -142,6 +142,16 @@ pub struct FileActivityEntry {
     pub timestamp: DateTime<Utc>,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+pub struct DecisionLogEntry {
+    pub id: i64,
+    pub session_id: String,
+    pub decision: String,
+    pub alternatives: Vec<String>,
+    pub reasoning: String,
+    pub timestamp: DateTime<Utc>,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
 #[serde(rename_all = "snake_case")]
 pub enum FileActivityAction {