feat(image): add Gemini Nano Banana 2.1 and deprecate gemini-3.1-flash-image

[Giom-V]

Summary

• Adds gemini-nano-banana-2.1 (Nano Banana 2.1) as the default high-efficiency image generation and conversational editing model in gemini-api-dev.
• Marks gemini-3.1-flash-image as legacy/deprecated (shutdown date October 29, 2026), replacing it with gemini-nano-banana-2.1.
• Documents key capabilities in references/migration.md:
  • 131,072 input token limit / 32,768 output token limit
  • Multi-image reference support (up to 14 reference images: character consistency for up to 4 characters, object fidelity for up to 10 objects)
  • Native Google Search and Image Search grounding
  • Configurable thinking levels (minimal, medium default, high)

[gemini-code-assist]

Code Review

This pull request updates the Gemini API skill documentation to deprecate the gemini-3.1-flash-image model and introduce its successor, gemini-nano-banana-2.1, along with its migration checklist and key features. One issue was identified in the migration guide where the documentation link for the new model page is missing the .md.txt extension, which is required for the agent to fetch the file correctly.