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/agent/agent.rs

@@ -566,7 +566,7 @@ pub async fn run(
 mod tests {
     use super::*;
     use async_trait::async_trait;
-    use std::sync::Mutex;
+    use parking_lot::Mutex;
 
     struct MockProvider {
         responses: Mutex<Vec<crate::providers::ChatResponse>>,
@@ -590,7 +590,7 @@ mod tests {
             _model: &str,
             _temperature: f64,
         ) -> Result<crate::providers::ChatResponse> {
-            let mut guard = self.responses.lock().unwrap();
+            let mut guard = self.responses.lock();
             if guard.is_empty() {
                 return Ok(crate::providers::ChatResponse {
                     text: Some("done".into()),