refactor(store): extract SQLite storage boundary

[0xjunha]

Summary

This PR extracts Darc’s SQLite storage layer into a dedicated darc-store crate and tightens the architecture boundary between indexing, storage, and querying.

Before this change, darc-query depended on darc-index to access SQLite schema/opening helpers, which made the read side depend on the write-side indexing crate. This PR moves the shared persistence concerns into darc-store, so darc-index can focus on archive ingestion and darc-query can read through the shared storage boundary.

Changes

• Add new darc-store workspace crate
  • owns SQLite schema and migrations
  • owns index DB open helpers
  • owns derived analytics/search row generation
  • owns evidence fields and file/tool access policies
  • owns storage test fixtures
• Update darc-index
  • remove direct ownership of SQLite schema/migration modules
  • keep ingestion, parsing, duplicate resolution, and index orchestration
  • write sessions/turns through store-owned insert helpers
• Update darc-query
  • remove dependency on darc-index
  • depend on darc-store for evidence fields, policies, and DB opening
• Keep raw schema SQL internal
  • make schema crate-private in darc-store
  • remove darc_index::schema re-export
  • hide derived insertion and turn-metric helpers behind store write APIs

Compatibility

This narrows internal Rust crate APIs by no longer exposing raw SQLite schema details outside the storage crate. CLI behavior and JSON query protocol behavior are unchanged.