fix(discord): use channel_id instead of sender for replies (fixes #483)

fix(misc): complete parking_lot::Mutex migration (fixes #505)

- DiscordChannel: store actual channel_id in ChannelMessage.channel
  instead of hardcoded "discord" string
- channels/mod.rs: use msg.channel instead of msg.sender for replies
- Migrate all std::sync::Mutex to parking_lot::Mutex:
  * src/security/audit.rs
  * src/memory/sqlite.rs
  * src/memory/response_cache.rs
  * src/memory/lucid.rs
  * src/channels/email_channel.rs
  * src/gateway/mod.rs
  * src/observability/traits.rs
  * src/providers/reliable.rs
  * src/providers/router.rs
  * src/agent/agent.rs
- Remove all .lock().unwrap() and .map_err(PoisonError) patterns
  since parking_lot::Mutex never poisons

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

src/observability/traits.rs

@@ -85,7 +85,7 @@ pub trait Observer: Send + Sync + 'static {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use std::sync::Mutex;
+    use parking_lot::Mutex;
     use std::time::Duration;
 
     #[derive(Default)]
@@ -96,12 +96,12 @@ mod tests {
 
     impl Observer for DummyObserver {
         fn record_event(&self, _event: &ObserverEvent) {
-            let mut guard = self.events.lock().unwrap();
+            let mut guard = self.events.lock();
             *guard += 1;
         }
 
         fn record_metric(&self, _metric: &ObserverMetric) {
-            let mut guard = self.metrics.lock().unwrap();
+            let mut guard = self.metrics.lock();
             *guard += 1;
         }
 
@@ -121,8 +121,8 @@ mod tests {
         });
         observer.record_metric(&ObserverMetric::TokensUsed(42));
 
-        assert_eq!(*observer.events.lock().unwrap(), 2);
-        assert_eq!(*observer.metrics.lock().unwrap(), 1);
+        assert_eq!(*observer.events.lock(), 2);
+        assert_eq!(*observer.metrics.lock(), 1);
     }
 
     #[test]