[Docs] Explain Fast mode and cost detail behavior

[roomote-community]

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

What changed

• document that Slack and Discord Fast mode can answer from conversation context, the Brain, and enabled deployment integrations before delegating execution work to a Roomote task
• explain that Cost Analytics consolidates detail rows by task, sums their cost, and displays Multiple for dimensions that vary within a task

Why this change was made

A review of user-facing changes merged during the previous 24 hours found these shipped behaviors were not yet explained in the public documentation.

Impact

Users can now understand what Fast mode can answer directly and correctly interpret consolidated Cost Analytics detail rows.

Screenshots

[roomote-community]

1 issue outstanding. See task

• apps/docs/cost-analytics.mdx:48 qualify the combined cost as limited to the selected chart detail.

_{Reviewed 04e1e3d}

[roomote-community]

The combined cost is only for the currently selected chart detail: getAnalyticsDetails filters by the active series, time bucket, and filters before aggregating by task. A task with usage across different buckets or series therefore appears with a partial cost here, not the task's total cost. Please qualify this as the cost within the selected detail view.