feat(agy): add gemini-3.8-flash default preference and fallback support

[matdev83]

Summary

• Add \google/gemini-3.8-flash\ to the top of \pickDefaultModel\ preferred list so sessions defaulting to auto pick the newly released model when discovered.
• Add \gemini-3.8-flash-high, \gemini-3.8-flash-medium, and \gemini-3.8-flash-low\ to \allbackModelIDs.
• Update \TestModelCatalogFallback\ and add \TestModelCatalogDiscoverGemini38Flash\ covering normalization and reasoning effort resolution.
• Update \README.md\ documentation for preferred default hierarchy.

Verification

• \go test ./...\ passed across all packages.
• \gofmt\ verified and clean.
• \scripts/check-release-clean.sh\ pre-commit and pre-push hooks passed.

[coderabbitai]

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