[cschanhniem]

Implements J2 from NEXT_100_PR_MAP: schema version migration guide.

What

• docs/SCHEMA_MIGRATION_GUIDE.md: 9 sections
• Versioning conventions table: MAJOR/MINOR/PATCH with artifact impact
• Adding a field (MINOR): two-step example with BatchOutcomeSummary
• Removing a field (MAJOR): two-step deprecate-then-remove
• Renaming a field (MAJOR): accept-both then remove
• Tightening validation (MAJOR): audit-migrate-then-schema PR order
• Loosening validation (MINOR): safe
• Schema ID stability: $id and prefix immutability, when to create new schema
• Migration notes table: placeholder for first breaking change
• Agent rules: MUST NOT change $id without review; MAY add optional fields

Why

Without a migration guide, every schema change is a guess about whether existing artifacts survive. The two-step rule (deprecate then remove) forces a window where old and new artifacts can coexist. The migration notes table creates an auditable record of breaking changes.

Disconfirming pass

• Docs-only PR; no code or schema changes
• No existing schema IDs changed
• No existing fields removed
• Policy is advisory; enforcement via code review and the agent MUST NOT rule