Add SQLMesh future reference track

[sabino]

Summary

• add a public-safe SQLMesh future reference map for later state/planner/cross-database design
• update PLAN.md with deferred SQLMesh adoption rules, validation gates, and risk notes
• update CHANGELOG.md for the planning note

Validation

• git diff --check HEAD~1..HEAD
• python scripts/check_public_safety.py
• python scripts/check_runtime_boundary.py

Review

• GPT-5.5 subagent PR-boundary review: no blocking or non-blocking findings