fix(channels): use platform message IDs to prevent duplicate memories

Fixes #430 - Prevents duplicate memories after restart by using platform message IDs instead of random UUIDs.

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

src/channels/slack.rs

@@ -160,8 +160,8 @@ impl Channel for SlackChannel {
                     last_ts = ts.to_string();
 
                     let channel_msg = ChannelMessage {
-                        id: Uuid::new_v4().to_string(),
-                        sender: channel_id.clone(),
+                        id: format!("slack_{channel_id}_{ts}"),
+                        sender: user.to_string(),
                         content: text.to_string(),
                         channel: "slack".to_string(),
                         timestamp: std::time::SystemTime::now()
@@ -252,4 +252,53 @@ mod tests {
         assert!(ch.is_user_allowed("U111"));
         assert!(ch.is_user_allowed("anyone"));
     }
+
+    // ── Message ID edge cases ─────────────────────────────────────
+
+    #[test]
+    fn slack_message_id_format_includes_channel_and_ts() {
+        // Verify that message IDs follow the format: slack_{channel_id}_{ts}
+        let ts = "1234567890.123456";
+        let channel_id = "C12345";
+        let expected_id = format!("slack_{channel_id}_{ts}");
+        assert_eq!(expected_id, "slack_C12345_1234567890.123456");
+    }
+
+    #[test]
+    fn slack_message_id_is_deterministic() {
+        // Same channel_id + same ts = same ID (prevents duplicates after restart)
+        let ts = "1234567890.123456";
+        let channel_id = "C12345";
+        let id1 = format!("slack_{channel_id}_{ts}");
+        let id2 = format!("slack_{channel_id}_{ts}");
+        assert_eq!(id1, id2);
+    }
+
+    #[test]
+    fn slack_message_id_different_ts_different_id() {
+        // Different timestamps produce different IDs
+        let channel_id = "C12345";
+        let id1 = format!("slack_{channel_id}_1234567890.123456");
+        let id2 = format!("slack_{channel_id}_1234567890.123457");
+        assert_ne!(id1, id2);
+    }
+
+    #[test]
+    fn slack_message_id_different_channel_different_id() {
+        // Different channels produce different IDs even with same ts
+        let ts = "1234567890.123456";
+        let id1 = format!("slack_C12345_{ts}");
+        let id2 = format!("slack_C67890_{ts}");
+        assert_ne!(id1, id2);
+    }
+
+    #[test]
+    fn slack_message_id_no_uuid_randomness() {
+        // Verify format doesn't contain random UUID components
+        let ts = "1234567890.123456";
+        let channel_id = "C12345";
+        let id = format!("slack_{channel_id}_{ts}");
+        assert!(!id.contains('-')); // No UUID dashes
+        assert!(id.starts_with("slack_"));
+    }
 }