Multi-modal support, type overhaul, and CI

[pint-drinker]

Summary

• Migrate Node SDK to multi-modal runs: new src/image.ts, expanded src/types.ts, and helper utilities in src/helpers.ts
• Consolidate types (remove src/types/* tree) and move error type to src/errors.ts; refactor client.ts, index.ts, stream.ts, and internal/http.ts to match
• Add GitHub Actions CI, Husky pre-commit/pre-push hooks, ESLint config, and tsc --noEmit typechecking
• New test suite covering body, client, content, polling, tool-response, and types
• Rewrite README and add MIGRATION.md documenting the API changes

Test plan

• pnpm typecheck passes
• pnpm test passes locally
• GitHub Actions CI green on the PR
• Smoke-test a multi-modal run against a real API key
• Verify MIGRATION.md steps actually work against a pre-migration consumer

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