feat(brain): add schema versioning and entity migration

[themusicman]

Summary

• Add schema versioning with integer version tracking and migration definitions
• Implement entity migration engine that transforms entities between schema versions
• Expose MigrateEntities action so the agent can trigger migrations via tool call

Detailed Changes

Schema Versioning (brain/schema.ex)

• Schemas now carry a version integer (defaults to 1)
• save_schema requires a migration map (from_version, to_version, field ops) when updating an existing schema
• Version validation ensures monotonic progression

Migration Engine (brain/migration.ex)

• Applies field-level operations: add_field, remove_field, rename_field, transform_field
• Processes all entities of a type in a single pass, writing updated TOML back to disk
• Stores migration history in brain/<type>/.migrations/ for auditability
• Dry-run mode for previewing changes without writing

MigrateEntities Action (actions/brain/migrate_entities.ex)

• New Jido action that wraps the migration engine
• Accepts type, workspace, and optional dry_run flag

Path Helpers (brain/paths.ex)

• Extracted shared path-building logic into a dedicated module
• Used by both Brain and Migration to resolve workspace/type/entity paths

Error Handling (actions/brain/helpers.ex)

• New error formatters for migration-specific failure modes

Agent Registration (agent.ex)

• MigrateEntities added to the agent's action list

Test Plan

• Schema version round-trips through save/load
• Migration validates version fields and field operations
• Entity migration applies add/remove/rename/transform correctly
• Migration history files are written and readable
• Dry-run mode returns preview without side effects
• Path helpers resolve correctly for nested workspace layouts
• MigrateEntities action integrates end-to-end in brain action tests