feat(knowledge-repository): implement candidate source foundation

[adamshen1007]

Summary

• add versioned Knowledge Repository and Candidate Source contracts
• add validated in-memory candidate source and deterministic repository snapshot
• add repository-backed query execution while preserving Milestone 05 compatibility
• preserve object identity and provenance across repository access
• document ADR-0010 and Milestone 06 architecture boundaries

Verification

• pnpm format:check
• pnpm lint
• pnpm build
• pnpm typecheck
• pnpm test (112 tests passed)

Deferred by design

Persistence, embeddings, vector search, semantic ranking, knowledge graph persistence, agents, Hermes, MCP integrations, and UI.