feat(discord): typing indicator, DM support, and online presence

[RatulMaharaj]

Three quality-of-life additions to the Discord trigger.

Typing indicator. Opt-in show_typing option (default false). When enabled, the bot posts POST /channels/{id}/typing as soon as a message passes the filters and re-fires it every 8 seconds (the indicator expires after ~10 s), stopping when the run finishes. Failures are swallowed — the indicator is cosmetic and never breaks a run. It always shows in the source channel, regardless of reply_channel. Documented as a poor fit with allow_silence, since typing followed by no message looks strange.

Direct messages. The gateway identify now requests the DIRECT_MESSAGES intent, and shouldHandle treats messages without a guild_id as DMs: the channels filter and require_mention gate are skipped (a DM addresses the bot by definition, matching the Slack and Telegram triggers), while from_users still applies. Each DM channel gets its own conversationKey, so DM history stays separate from channel history.

Online presence. The identify payload now carries an explicit presence: online, so the bot reads as online for as long as the gateway connection is up.

Also updates docs/discord.mdx (new Direct messages section, show_typing in the example and observer guidance), the config schema descriptions, and the regenerated schema/agent.json. Unit tests cover the typing loop and the DM filter exemptions.

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