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>

tests/whatsapp_webhook_security.rs

@@ -72,7 +72,9 @@ fn whatsapp_signature_rejects_tampered_body() {
 
     // Tampered body should be rejected even with valid-looking signature
     assert!(!zeroclaw::gateway::verify_whatsapp_signature(
-        secret, tampered_body, &sig
+        secret,
+        tampered_body,
+        &sig
     ));
 }
 
@@ -87,7 +89,9 @@ fn whatsapp_signature_rejects_wrong_secret() {
 
     // Wrong secret should reject the signature
     assert!(!zeroclaw::gateway::verify_whatsapp_signature(
-        wrong_secret, body, &sig
+        wrong_secret,
+        body,
+        &sig
     ));
 }