feat(ai): add Google image generation

[opencode-agent]

Summary

• add Google.configure(...).image(modelID) backed by Gemini generateContent for current Nano Banana image models
• filter interim thought: true image parts while preserving ordered candidate/part metadata, text boundaries, thought flags, and signatures without duplicating image bytes
• reject unsupported imagen-* model IDs locally and use valid REST thinkingLevel enum literals
• normalize token counts while preserving the complete raw Google usage payload, including serviceTier
• include finish reasons, prompt feedback, safety details, and candidate context in no-image errors
• preserve Google API-key/env auth and merged HTTP headers, query parameters, and body overlays

Tests

• bun x prettier --check README.md src/protocols/google-images.ts test/image.test.ts test/provider/google-images.recorded.test.ts src/providers/google.ts
• GOMAXPROCS=4 bun typecheck
• bun run build
• bun test test/image.test.ts --timeout 30000 --only-failures (6 passed)
• bun test --timeout 30000 --only-failures (357 passed, 29 skipped)
• RECORDED_PREFIX=google-images bun test test/provider/google-images.recorded.test.ts --timeout 30000 --only-failures (1 passed)
• scanned the cassette against GOOGLE_GENERATIVE_AI_API_KEY and credential header names; no secret material is present

Live evidence

• generated one 412,499-byte JPEG through the actual Google route
• decoded and visually inspected the result: a centered blue circle on a white background, matching the prompt
• verified JPEG format and 1024x1024 dimensions
• retained the existing cassette because the request shape did not change; replay succeeds without re-recording

Limitations

• this adapter supports Gemini generateContent image models only; Imagen uses a different API and is rejected locally
• Google generateContent does not expose exact pixel dimensions or a requested image count, so portable size and count are rejected with typed invalid-request errors
• output format is provider-selected; the returned inline-data MIME type is preserved
• scope is single-shot direct Image.generate; no conversational continuation path is added

Requested by: @rekram1-node (Aiden Cline via Slack)