[Docs] Sync recent user-facing product behavior

[roomote-community]

​Created by Roomote. View the task or mention @roomote for follow-up asks.

What changed

• Documented the new Cost Analytics source dimension and Fast delegated-task model selection.
• Clarified task-linked GitHub CI failure notifications, Brain pull-request content, and configuration-alert delivery fallbacks.

Why this change was made

Several user-facing behaviors merged without matching public documentation. This keeps the documented workflows aligned with the behavior currently shipped on develop.

Impact

Users and deployment admins can now find accurate guidance for these analytics, model-selection, source-control, Brain, and automation behaviors. There is no runtime behavior change.

Screenshots

[roomote-community]

No code issues found. See task

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