Add GitHub Models retirement banners and legacy labeling (#494)

[jongalloway]

Summary

Adds consistent GitHub Models retirement messaging so no learner-facing page presents it as the default/recommended provider without a warning. Resolves #494.

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

Changes

Retirement banners / legacy labeling (retires 2026-07-30, brownouts July 16 & 23, pointing to Azure AI Foundry) added to:

• Root README.md — top [!IMPORTANT] banner + legacy label on the GitHub Models tech bullet
• Part 1 - Setup — top retirement banner
• Part 4 - Azure OpenAI — retirement banner + legacy label on the provider bullet
• Part 2 / Part 5 / Part 6 GenAiLab READMEs — [!WARNING] under the "Using GitHub Models" section (was presented first, without warning)
• .github/copilot-instructions.md — GitHub Models marked legacy dev-only, retiring
• .github/.devcontainer/README.md — legacy note on the GitHub-account container criterion

Acceptance

• ✅ No page presents GitHub Models as the recommended/default provider without a retirement warning
• ✅ Brownout (July 16 & 23) and full-retirement (July 30) dates surfaced where relevant
• ✅ Prereq lists already mark the GitHub account as optional/legacy (root + Part 1)

Notes

• Several edited pages live in the old-structure parts (Part 2/4/5/6 folders); banners are added now because the retirement is time-sensitive (pre-July brownouts). The folder renumber/dedupe happens later in the restructure pass (Restructure & renumber; remove Products Page; advanced MCP optional #503).
• The new hands-on parts (2–5) and Part 5 providers doc already carried this messaging.