[Feat] Add Goal Mode across web and Slack

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• start new web tasks in Goal Mode with /goal <objective>
• enable Goal Mode from active Slack task threads and direct messages with thread-aware message commands
• share the existing activation lifecycle between Slack and Discord, including rollback and duplicate-delivery protection
• extend the existing Goal Mode guide with web-launcher and Slack usage while preserving its lifecycle and completion guidance

Why this change was made

Goal Mode was discoverable only inside existing web tasks and Discord. The web launcher could not create a goal-driven task, and Slack had no thread-safe way to activate the mode.

Impact

Users can start Goal Mode directly from the web launcher or activate it in an existing Slack conversation. Later top-level Slack DM commands resolve the active task by DM channel and keep replies attached to the original task thread.

Screenshots

[roomote-community]

No code issues found. See task

_{Reviewed 3511e01}