Author a first-party Nx migration

Companion files in this directory:

• runtime-contract.md: how nx migrate consumes every migrations.json key. Read it before wiring an entry; most authoring mistakes are wrong assumptions about this contract.
• deprecated-patterns.md: patterns to never reproduce, with recognition signatures. Read it before copying from an existing migration or from git history.
• templates/: entry shapes, spec skeleton, and the two .md genres.

1. Decompose the change into migration needs

Enumerate every breaking or behavior-changing item in the change (upstream changelog, upstream migration guide, upstream repo's own migrations directory, or the Nx-internal change itself). Classify each item with exactly one treatment: