feat(sdk): expose evaluate api

[christso]

Summary

• Re-export the core-owned evaluate() API and related option/result types from @agentv/sdk.
• Add @agentv/core as a one-way SDK dependency while keeping @agentv/core free of any @agentv/sdk package dependency.
• Remove core test imports from packages/sdk/src by replacing them with local contract fixtures.
• Update SDK README and public TypeScript SDK docs to use import { evaluate } from '@agentv/sdk'.

Dependency graph

• @agentv/sdk -> @agentv/core is now explicit.
• @agentv/core -> @agentv/sdk remains absent in packages/core/package.json.
• Added packages/sdk/test/package-graph.test.ts to guard against core source/test imports from SDK source.

Verification

• bun --filter @agentv/sdk test
• bun --filter @agentv/sdk typecheck
• bun --filter @agentv/sdk build
• bun --filter @agentv/core test test/evaluation/loaders/ts-eval-loader.test.ts
• bun --filter @agentv/core test test/evaluation/graders.test.ts
• bun --filter @agentv/core test test/evaluation/graders/prompt-resolution.test.ts
• bunx biome check packages/sdk/src/index.ts packages/sdk/test/evaluate-export.test.ts packages/sdk/test/package-graph.test.ts packages/core/test/evaluation/graders.test.ts packages/core/test/evaluation/graders/prompt-resolution.test.ts packages/core/test/evaluation/loaders/fixtures/sdk-define-eval.eval.ts packages/core/test/fixtures/test-define-grader.ts

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