xtask subtree rules

Packaging, migration, validation, and headless commands must be deterministic and scriptable.
Packaging is a slice or candidate gate, not an inner-loop command.
Package tests use compact fixtures and must not require the editor crate.
Repeated package builds must be idempotent.
Use the isolated package lane; do not invalidate the fast editor lane.
Run package-focused tests before any full-workspace gate when packaging changes.