[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 proactive provider usage-limit warnings in the Slack Manager Channel
• documented additional trusted GitHub App slugs and Fast PR-feedback actions
• clarified how Fast task input replies and zero-alert Dependabot scans behave

Why this change was made

Several user-facing behaviors merged recently without complete public guidance. These updates align the public docs with the current shipped behavior and operator configuration surface.

Impact

Operators and users can now find the prerequisites, configuration, and expected interaction behavior for provider limit alerts, managed GitHub App identities, Fast follow-ups, and Dependabot triage reports.

Screenshots

[roomote-community]

No code issues found. See task

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