[Feat] Add Goal Mode command to Discord

[roomote-community]

​Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• Register /goal objective:<objective> as a Discord application command using the same “Keep working toward an objective across multiple turns” description as the web command picker.
• Use that canonical description in Discord /help and the Discord documentation.
• Enable Goal Mode for the active task in the current Discord task thread or DM without creating or resuming another task.
• Preserve atomic Goal Mode activation by rolling back when prompt delivery or activation commit fails.

Why this change was made

Goal Mode was available from the web task composer but missing from Discord's command picker. This adds feature parity while making the command's multi-turn behavior clear and consistent across discovery surfaces.

Impact

Linked Discord users can run /goal inside an active Roomote task conversation to keep working toward an objective across multiple turns. Running it outside an active task returns guidance instead of starting or selecting a different task.

[roomote-community]

No new code issues found. See task

• Discord accepts the /goal command with objective.max_length capped at 6,000 characters. (packages/communication/src/discord-provider.ts:1230)

_{Reviewed bca82a3}