fix(memory): prevent autosave key collisions across runtime flows

Fixes #221 - SQLite Memory Override bug.

This PR resolves memory overwrite behavior in autosave paths by replacing fixed memory keys with unique keys, and improves short-horizon recall quality in channel runtime.

**Root Cause**
SQLite memory uses a unique constraint on `memories.key` and writes with `ON CONFLICT(key) DO UPDATE`.
Several autosave paths reused fixed keys (or sender-stable keys), so newer messages overwrote earlier conversation entries.

**Changes**
- Channel runtime: autosave key changed from `channel_sender` to `channel_sender_messageId`
- Added memory-context injection before provider calls (aligned with agent loop behavior)
- Agent loop: autosave keys changed from fixed `user_msg`/`assistant_resp` to UUID-suffixed keys
- Gateway: Webhook/WhatsApp autosave keys changed to UUID-suffixed keys

All CI checks passing.

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

src/providers/reliable.rs

@@ -424,10 +424,7 @@ mod tests {
             1,
         );
 
-        let messages = vec![
-            ChatMessage::system("system"),
-            ChatMessage::user("hello"),
-        ];
+        let messages = vec![ChatMessage::system("system"), ChatMessage::user("hello")];
         let result = provider
             .chat_with_history(&messages, "test", 0.0)
             .await