Document model fallback lists, CIMD OAuth, and sandbox network isolation changes (v1.0.83) (#2953)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/src/content/docs/learning-hub/building-custom-agents.md

@@ -3,7 +3,7 @@ title: 'Building Custom Agents'
 description: 'Learn how to create specialized GitHub Copilot agents with custom personas, tool integrations, and domain expertise.'
 authors:
   - GitHub Copilot Learning Hub Team
-lastUpdated: 2026-08-09
+lastUpdated: 2026-09-05
 estimatedReadingTime: '10 minutes'
 tags:
   - agents
@@ -73,6 +73,17 @@ tools: ['codebase', 'terminal', 'github']
 
 **model** (recommended): The AI model that powers the agent. Choose based on the complexity of the task—use more capable models for nuanced reasoning.
 
+> **Model fallback lists (v1.0.83+)**: `model` can list several models, tried in order until one is available to you — useful if your preferred model is temporarily rate-limited or not enrolled for your account. Pair it with `model-policy: required` to keep the agent restricted to that list even if the user tries to switch models mid-session:
+>
+> ```yaml
+> ---
+> name: 'Security Reviewer'
+> model: [Claude Sonnet 4.5, Claude Sonnet 4, GPT-5]
+> model-policy: required
+> tools: ['codebase', 'terminal', 'github']
+> ---
+> ```
+
 **reasoningEffort** *(v1.0.66+)*: Override the reasoning effort level for this agent. Accepted values are `low`, `medium`, and `high`. This lets you pin specific agents to a cost/quality tradeoff regardless of the user's global setting — for example, a quick code-formatting agent can use `low` effort, while a security reviewer uses `high`:
 
 ```yaml