feat(discord): binding reload without a restart, and a per-user channel allowlist (#1509)

`reload` validates the binding file and sends SIGHUP to the live owner;
the running connector re-reads it and swaps guildName, channels, users
and limits in place. name, seat, guildId, botUserId, tokenFile, engine
and context are fixed for the life of the process; a change there, an
invalid file or a channel outside the guild refuses the reload and keeps
the old binding. Every attempt is one line in reloads.jsonl. The service
unit maps `systemctl --user reload` to the same signal.

A user entry may carry `channels`, an allowlist of listed channel ids;
absent means every listed channel. Outside the list the message is
dropped as channel-not-for-user; threads count as their parent.

Suite 41/41, 101 node tests. QUEUE rows 19 and 20 opened.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/discord/fixtures/binding.example.json

@@ -11,7 +11,8 @@
     { "id": "100000000000000011", "name": "general", "mode": "mention" }
   ],
   "users": [
-    { "id": "100000000000000100", "name": "owner" }
+    { "id": "100000000000000100", "name": "owner" },
+    { "id": "100000000000000101", "name": "guest", "channels": ["100000000000000011"] }
   ],
   "engine": { "provider": "zai", "model": "glm-5.3", "thinking": "high" },
   "limits": {