CAP-001 Phase 2D provider hardening

[csailer]

Summary

• Completes CAP-001 Phase 2D provider hardening and compatibility closure.
• Adds regression coverage for existing DB reuse, repeated export/verify reads, malformed data behavior, legacy DB upgrade, and deterministic rehydration.
• Hardens internal SQLite provider health for connectivity, migration/schema readiness, writable state, and closed provider lifecycle.
• Finalizes CAP-001 storage abstraction documentation and phase completion assessment.

Validation

• go test ./...
• go vet ./...
• go build -o bin/yanzi ./cmd/yanzi
• make docs-build
• Fresh DB representative CLI workflow: project create/list/use/current, capture, list, show, checkpoint create/list, rehydrate, verify, chain, export markdown/json/html
• Repeated CLI workflow: verify, rehydrate, export json
• Upgraded DB compatibility: legacy SQLite upgrade and rehydration regression coverage

Notes

• SQLite remains the only provider.
• No provider config switching, runtime, REST, federation, MCP, schema redesign, or user-visible behavior changes are introduced.