Add GitHub Models retirement banners and legacy labeling

[jongalloway]

Summary

Add clear, consistent messaging that GitHub Models is retiring July 30, 2026 so learners aren't surprised and know to use Azure AI Foundry instead.

Reference: https://github.blog/changelog/2026-07-01-github-models-is-being-fully-retired-on-july-30-2026/

Tasks

• Add a retirement banner (note/callout) to the root README.md
• Add a retirement banner to Part 1 setup
• Wherever GitHub Models is mentioned as a provider, mark it as legacy / retiring and point to Azure AI Foundry
• Mention the brownout dates (July 16 & 23) and full retirement (July 30) where relevant
• Update prerequisite lists to make the GitHub account optional (legacy fallback) rather than required

Acceptance criteria

• No page presents GitHub Models as the recommended/default provider without a retirement warning.