feat(discord): opt-in Discord Activities widget support (#107442)

Adds a Discord Activities integration so an agent can show a self-contained
HTML widget to Discord users, opened as a sandboxed Activity inside the client.
Off by default: routes, the discord_widget tool, and the launch handler register
only when channels.discord.activities is configured. OAuth identifies the user
and gates on the account allowlist; widget lookup is capability- or
instance-validated; token exchange is rate-limited; widget HTML runs in a
no-network sandboxed iframe.

docs/docs.json

@@ -1143,6 +1143,7 @@
                 "group": "Mainstream messaging",
                 "pages": [
                   "channels/discord",
+                  "channels/discord-activities",
                   "channels/slack",
                   "channels/telegram",
                   "channels/whatsapp",