feat: Add SQLite fallback mode for Docker-free setup

[erwinh22]

Problem

Currently, ACE requires Docker to run Qdrant for vector storage. This creates friction for:

• Developers without Docker installed
• Quick experimentation/evaluation
• Environments where Docker isn''t available

Proposed Solution

Add an optional SQLite-based vector storage backend using sqlite-vss or chromadb as an alternative to Qdrant.

Requirements

• Zero external dependencies for basic usage
• Same API surface as Qdrant backend
• Clear documentation on performance trade-offs
• Easy migration path from SQLite to Qdrant

Non-Goals

• Full feature parity (enterprise features can require Qdrant)
• Production-scale performance

Alternatives Considered

• Qdrant Cloud free tier (still requires account setup)
• In-memory storage (loses persistence)

Additional Context

Raised during adoption readiness review.