feat(memory): add message pagination support

- get_messages_paginated(session_id, limit, offset) for batch loading
- count_messages(session_id) for total count queries
- Enables frontend to load messages progressively instead of all-at-once

crates/zclaw-memory/src/store.rs

@@ -324,6 +324,46 @@ impl MemoryStore {
         Ok(messages)
     }
 
+    /// Get messages for a session with pagination
+    pub async fn get_messages_paginated(
+        &self,
+        session_id: &SessionId,
+        limit: u32,
+        offset: u32,
+    ) -> Result<Vec<Message>> {
+        let session_str = session_id.to_string();
+
+        let rows = sqlx::query_as::<_, (String,)>(
+            "SELECT content FROM messages WHERE session_id = ? ORDER BY seq LIMIT ? OFFSET ?"
+        )
+        .bind(&session_str)
+        .bind(limit)
+        .bind(offset)
+        .fetch_all(&self.pool)
+        .await
+        .map_err(|e| ZclawError::StorageError(e.to_string()))?;
+
+        let messages = rows
+            .into_iter()
+            .filter_map(|(content,)| serde_json::from_str(&content).ok())
+            .collect();
+        Ok(messages)
+    }
+
+    /// Count messages in a session
+    pub async fn count_messages(&self, session_id: &SessionId) -> Result<u64> {
+        let session_str = session_id.to_string();
+        let count: i64 = sqlx::query_scalar(
+            "SELECT COUNT(*) FROM messages WHERE session_id = ?"
+        )
+        .bind(&session_str)
+        .fetch_one(&self.pool)
+        .await
+        .map_err(|e| ZclawError::StorageError(e.to_string()))?;
+
+        Ok(count as u64)
+    }
+
     // === KV Store ===
 
     /// Store a key-value pair for an agent