feat(discord): add set-presence action for bot activity and status

Bridge the agent tools layer to the Discord gateway WebSocket via a new
gateway registry, allowing agents to set the bot's activity and online
status. Supports playing, streaming, listening, watching, custom, and
competing activity types. Custom type uses activityState as the sidebar
text; other types show activityName in the sidebar and activityState in
the flyout. Opt-in via channels.discord.actions.presence (default false).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/channels/discord.md

@@ -100,7 +100,7 @@ In **Bot** → **Privileged Gateway Intents**, enable:
 - **Message Content Intent** (required to read message text in most guilds; without it you’ll see “Used disallowed intents” or the bot will connect but not react to messages)
 - **Server Members Intent** (recommended; required for some member/user lookups and allowlist matching in guilds)
 
-You usually do **not** need **Presence Intent**.
+You usually do **not** need **Presence Intent**. Setting the bot's own presence (`setPresence` action) uses gateway OP3 and does not require this intent; it is only needed if you want to receive presence updates about other guild members.
 
 ### 3) Generate an invite URL (OAuth2 URL Generator)
 
@@ -278,6 +278,7 @@ Outbound Discord API calls retry on rate limits (429) using Discord `retry_after
         voiceStatus: true,
         events: true,
         moderation: false,
+        presence: false,
       },
       replyToMode: "off",
       dm: {
@@ -353,6 +354,7 @@ ack reaction after the bot replies.
   - `channels` (create/edit/delete channels + categories + permissions)
   - `roles` (role add/remove, default `false`)
   - `moderation` (timeout/kick/ban, default `false`)
+  - `presence` (bot status/activity, default `false`)
 - `execApprovals`: Discord-only exec approval DMs (button UI). Supports `enabled`, `approvers`, `agentFilter`, `sessionFilter`.
 
 Reaction notifications use `guilds.<id>.reactionNotifications`:
@@ -412,6 +414,7 @@ Allowlist notes (PK-enabled):
 | events         | enabled  | List/create scheduled events       |
 | roles          | disabled | Role add/remove                    |
 | moderation     | disabled | Timeout/kick/ban                   |
+| presence       | disabled | Bot status/activity (setPresence)  |
 
 - `replyToMode`: `off` (default), `first`, or `all`. Applies only when the model includes a reply tag.
 
@@ -460,6 +463,7 @@ The agent can call `discord` with actions like:
 - `searchMessages`, `memberInfo`, `roleInfo`, `roleAdd`, `roleRemove`, `emojiList`
 - `channelInfo`, `channelList`, `voiceStatus`, `eventList`, `eventCreate`
 - `timeout`, `kick`, `ban`
+- `setPresence` (bot activity and online status)
 
 Discord message ids are surfaced in the injected context (`[discord message id: …]` and history lines) so the agent can target them.
 Emoji can be unicode (e.g., `✅`) or custom emoji syntax like `<:party_blob:1234567890>`.