Docs updates for E/A#94602 auto-add new agent as full copilot

[MelvinBot]

Explanation of Change

App #94602 changes agent creation so that a newly created agent is now automatically sent as a full-access Copilot of its creator's account, letting it act on the creator's behalf without the user adding it as a Copilot by hand. The create screen also now shows a note explaining this before the agent is created.

The existing help article Create and Use Agents described this as an optional / manual action ("You can also add an agent as a Copilot on your own account"). This PR updates the article to reflect the new automatic behavior:

• Intro: Clarifies that creating an agent automatically adds it as a full-access Copilot on your account, and points to Copilot: Delegated Access under Account > Security to review or remove that access.
• How to create an Agent: Adds a step noting the on-screen note above the Create agent button.
• What happens after you create an agent: Changes the "You can optionally add…" bullet to state the agent is automatically added as a full-access Copilot on your account.
• FAQ: Reworks "Can I add an agent as a Copilot on my own account?" to "Is an agent added as a Copilot on my own account?" and answers that it happens automatically on creation.
• Metadata: Updates the description to match the new behavior.

All UI labels (Copilot: Delegated Access, Account > Security, Create agent, New agent) were verified against the existing Copilot help articles and the PR's copy for exact capitalization.

Fixed Issues

$ https://github.com/Expensify/Expensify/issues/651811

Tests

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QA

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@yuwenmemon, please review the files changed and confirm they reflect the current behavior. Then mark this PR Ready for review.