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/observability/mod.rs

@@ -22,7 +22,10 @@ pub fn create_observer(config: &ObservabilityConfig) -> Box<dyn Observer> {
             ) {
                 Ok(obs) => {
                     tracing::info!(
-                        endpoint = config.otel_endpoint.as_deref().unwrap_or("http://localhost:4318"),
+                        endpoint = config
+                            .otel_endpoint
+                            .as_deref()
+                            .unwrap_or("http://localhost:4318"),
                         "OpenTelemetry observer initialized"
                     );
                     Box::new(obs)