feat: integrate DataLink semantic service

[iancaoo]

Summary

This PR introduces DataLink, DataFoundry's first-party semantic graph capability for enterprise data analysis. It connects physical schemas and data profiles with business concepts, entities, join paths, and confidence-scored relationships, giving the Data Agent richer context than schema inspection alone.

• add the DataLink semantic service under services/datalink
• expose semantic retrieval to agents through MCP and graph management through REST
• add opt-in DATALINK_ENABLED orchestration for development and formal deployment
• register a managed builtin-datalink resource with live health reporting while preserving externally configured services
• support shared DataFoundry model settings and dedicated DATALINK_* overrides
• add portable Python dependency management, service CI gates, health checks, and bilingual documentation

Feature value

DataLink strengthens DataFoundry in three areas:

1. Business-aware grounding: maps tables and columns to concepts and entities instead of relying only on physical names.
2. Relationship-aware analysis: exposes joinable paths, semantic equivalence, profiles, and confidence signals to reduce guesswork in complex multi-table tasks.
3. Governed first-party integration: keeps semantic retrieval inside DataFoundry's managed resource, audit, configuration, and deployment boundaries.

Deployment behavior

• default DATALINK_ENABLED=false: existing Web + API topology, with no Python or uv requirement
• DATALINK_ENABLED=true: npm run dev and npm run start additionally launch DataLink MCP on 8080 and REST on 8081
• split start:* commands remain available for existing process supervisors
• external DataLink-compatible services remain supported

Verification

• npm run build
• npm run test:web (586 passed)
• npm run lint:datalink
• npm run test:datalink (211 passed, 2 skipped)
• npm run build:datalink
• npm run test:datalink-stack
• node scripts/smoke-builtin-datalink.mjs
• node scripts/smoke-config-api.mjs
• npm run smoke:docs
• npm run docs:build
• live API + DataLink MCP/REST startup, managed-resource discovery, health checks, and shutdown cleanup

All GitHub Actions checks pass.

Scope

This feature keeps DataFoundry's current process-based deployment model. Docker, Kubernetes, multi-tenancy changes, and a language rewrite are intentionally out of scope.