Add memory migration API

[gharibyan]

Summary

• Add agent.memory.migrate() for source-agnostic import of caller-mapped memories and events.
• Add migration input/result types, validation, scope resolution, synthetic source events, source links, embeddings, and skipExisting handling.
• Document migration usage in the root and package READMEs.

Validation

• pnpm test
• pnpm lint
• pnpm pack:check
• git diff --check