[docs] Add CIDER_AGENT.md

This file provides essential instructions and troubleshooting steps for
AI agents operating within the Fuchsia Cog workspace. It is intended to
serve as a quick reference guide to ensure agents configure the
workspace properly and use the correct commands for tasks.

Change-Id: Ibb1f70c0f741ed7943c9d88f5175a7044843f0c2
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Fuchsia-Auto-Submit: Puneetha Ramachandra <puneetha@google.com>
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Owners-Override: Adam Barth <abarth@google.com>
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Commit-Queue: Puneetha Ramachandra <puneetha@google.com>