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/channels/email_channel.rs

@@ -17,7 +17,7 @@ use serde::{Deserialize, Serialize};
 use std::collections::HashSet;
 use std::io::Write as IoWrite;
 use std::net::TcpStream;
-use std::sync::Mutex;
+use parking_lot::Mutex;
 use std::time::{Duration, SystemTime, UNIX_EPOCH};
 use tokio::sync::mpsc;
 use tokio::time::{interval, sleep};
@@ -415,7 +415,7 @@ impl Channel for EmailChannel {
                             let mut seen = self
                                 .seen_messages
                                 .lock()
-                                .expect("seen_messages mutex should not be poisoned");
+                ;
                             if seen.contains(&id) {
                                 continue;
                             }