Add th diff schema change classification and migration hints

[snissn]

Background

Users need clear visibility into schema changes and migration risk before regenerating/redeploying.

Goal

Introduce schema diff tooling with migration guidance.

Scope

• New command: th diff <oldSchema> <newSchema>.
• Classify changes as: additive, breaking, or behavior-changing.
• Output suggested next actions (regen only, redeploy required, migration recommended).
• Optional machine-readable JSON mode for CI.

Implementation Notes

• Build on THS semantics (collections, fields, rules, indexes).
• Ensure deterministic output ordering for CI diffs.

Acceptance Criteria

• Diff output is stable and understandable for common changes.
• Breaking changes are explicitly flagged.
• CI can consume JSON mode for policy checks.