fix(discord): address review feedback on empty allowed_channels behavior

- docs: clarify "channels gate first" → "channels must be configured first"
- discord.rs: add debug! log on early return when allowed_channels is empty
- main.rs: downgrade startup log from error! to warn! (bot still starts intentionally)
- main.rs: add missing `warn` to tracing imports (fixes compile error)

Author: JARVIS-coding-Agent

docs/discord-bot-howto.md

@@ -77,7 +77,7 @@ allowed_channels = ["your-channel-id-from-step-6"]
 |---|---|---|
 | empty | empty | **No channels, no users — bot ignores all messages** |
 | set | empty | Only these channels, all users |
-| empty | set | **Bot ignores all messages** (channels gate first) |
+| empty | set | **Bot ignores all messages** (channels must be configured first) |
 | set | set | **AND** — must be in allowed channel AND allowed user |
 
 - Empty `allowed_channels` = bot will not respond anywhere (secure by default)

src/discord.rs

@@ -213,6 +213,7 @@ impl EventHandler for Handler {
 
         let channel_id = msg.channel_id.get();
         if self.allowed_channels.is_empty() {
+            debug!("allowed_channels is empty — ignoring message in channel {}", channel_id);
             return;
         }
         let in_allowed_channel = self.allowed_channels.contains(&channel_id);

src/main.rs

@@ -16,7 +16,7 @@ use serenity::prelude::*;
 use std::collections::HashSet;
 use std::path::PathBuf;
 use std::sync::Arc;
-use tracing::{error, info};
+use tracing::{error, info, warn};
 
 #[derive(Parser)]
 #[command(name = "openab")]
@@ -149,7 +149,7 @@ async fn main() -> anyhow::Result<()> {
                 let allowed_channels =
                     parse_id_set(&discord_cfg.allowed_channels, "discord.allowed_channels")?;
                 if allowed_channels.is_empty() {
-                    error!("no allowed_channels configured — bot will not respond to any messages. Add at least one channel ID to [discord] allowed_channels.");
+                    warn!("no allowed_channels configured — bot will not respond to any messages. Add at least one channel ID to [discord] allowed_channels.");
                 }
                 let allowed_users = parse_id_set(&discord_cfg.allowed_users, "discord.allowed_users")?;
                 let trusted_bot_ids = parse_id_set(&discord_cfg.trusted_bot_ids, "discord.trusted_bot_ids")?;