feat(knowledgebase): add local backend

[wucm667]

Summary

• add an in-memory local knowledgebase backend
• wire ktypes.LocalBackend into NewKnowledgeBase
• support adding text, files, and directories, with deterministic lexical search by default
• support optional custom embedding-based ranking through a lightweight backend embedder interface

Tests

• GOCACHE=/tmp/veadk-go-build-cache go test -mod=readonly ./knowledgebase/backend/local_knowledge_backend
• GOCACHE=/tmp/veadk-go-build-cache go test -mod=readonly -race ./knowledgebase/backend/local_knowledge_backend

Notes

• go test -mod=readonly ./knowledgebase could not be completed in the local sandbox because the existing ADK dependency needed module-cache writes/download access.