[Docs] Align public docs with recent product changes

[roomote-community]

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

What changed

• expand Models guidance with an orchestration environment-variable example and explicit Same as coding model inheritance behavior
• explain how Fast-mode inference appears in Cost Analytics
• align the Brain guide with its simplified status and source views and full paginated corpus browser

Why this change was made

The user-facing changes merged during the previous 24 hours were reviewed against their shipped behavior. These documentation gaps would otherwise leave admins with stale setup and product guidance.

Impact

Public documentation now accurately describes the current Models, Cost Analytics, and Brain experiences. Internal reliability changes from the same review window remain intentionally undocumented.

[roomote-community]

No new code issues found. See task

_{Reviewed 90646a4}