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/providers/reliable.rs

@@ -461,7 +461,7 @@ mod tests {
     /// Mock that records which model was used for each call.
     struct ModelAwareMock {
         calls: Arc<AtomicUsize>,
-        models_seen: std::sync::Mutex<Vec<String>>,
+        models_seen: parking_lot::Mutex<Vec<String>>,
         fail_models: Vec<&'static str>,
         response: &'static str,
     }
@@ -476,7 +476,7 @@ mod tests {
             _temperature: f64,
         ) -> anyhow::Result<String> {
             self.calls.fetch_add(1, Ordering::SeqCst);
-            self.models_seen.lock().unwrap().push(model.to_string());
+            self.models_seen.lock().push(model.to_string());
             if self.fail_models.contains(&model) {
                 anyhow::bail!("500 model {} unavailable", model);
             }
@@ -729,7 +729,7 @@ mod tests {
         let calls = Arc::new(AtomicUsize::new(0));
         let mock = Arc::new(ModelAwareMock {
             calls: Arc::clone(&calls),
-            models_seen: std::sync::Mutex::new(Vec::new()),
+            models_seen: parking_lot::Mutex::new(Vec::new()),
             fail_models: vec!["claude-opus"],
             response: "ok from sonnet",
         });
@@ -764,7 +764,7 @@ mod tests {
         let calls = Arc::new(AtomicUsize::new(0));
         let mock = Arc::new(ModelAwareMock {
             calls: Arc::clone(&calls),
-            models_seen: std::sync::Mutex::new(Vec::new()),
+            models_seen: parking_lot::Mutex::new(Vec::new()),
             fail_models: vec!["model-a", "model-b", "model-c"],
             response: "never",
         });