Add AI model selection documentation for Copilot coding agent (#57444)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: PickHub <6632368+PickHub@users.noreply.github.com>
Co-authored-by: Daniel Klemm <pickhub@github.com>
Co-authored-by: Isaac Brown <101839405+isaacmbrown@users.noreply.github.com>
Co-authored-by: Jon Caveman <jcaveman@github.com>
Co-authored-by: hubwriter <hubwriter@github.com>

Author: 5 people

content/copilot/concepts/agents/coding-agent/about-coding-agent.md

@@ -138,6 +138,7 @@ Users can include hidden messages in issues assigned to {% data variables.produc
 * **{% data variables.product.prodname_copilot_short %} doesn't account for content exclusions**. Content exclusions allow administrators to configure {% data variables.product.prodname_copilot_short %} to ignore certain files. When using {% data variables.copilot.copilot_coding_agent %}, {% data variables.product.prodname_copilot_short %} will not ignore these files, and will be able to see and update them. See [AUTOTITLE](/copilot/managing-copilot/configuring-and-auditing-content-exclusion/excluding-content-from-github-copilot).
 * **{% data variables.copilot.copilot_coding_agent %} does not apply the "Suggestions matching public code" policy**. {% data variables.product.prodname_copilot_short %} may still produce suggestions matching public code, even if the "Suggestions matching public code" policy is set to "Block." If this happens, {% data variables.product.prodname_copilot_short %} will not provide code references pointing to the original source of the code. See [AUTOTITLE](/copilot/managing-copilot/managing-copilot-as-an-individual-subscriber/managing-your-copilot-plan/managing-copilot-policies-as-an-individual-subscriber#enabling-or-disabling-suggestions-matching-public-code).
 * **{% data variables.copilot.copilot_coding_agent %} only works with repositories hosted on {% data variables.product.github %}**. If your repository is stored using a different code hosting platform, {% data variables.product.prodname_copilot_short %} won't be able to work on it.
+* **You cannot change the AI model used by {% data variables.copilot.copilot_coding_agent %}**. The model is determined by {% data variables.product.company_short %} and does not inherit the model selected for {% data variables.copilot.copilot_chat %}.
 
 ## Hands-on practice