mops-cli: add explicit check → build workflow in Enhanced migrations section

[marc0olo]

Problem

The Enhanced migrations section explains that mops check and mops build automatically inject --enhanced-migration, and that migration files go in the chain directory. But it does not explicitly state the two-step workflow agents should follow:

1. Run mops check --fix (or mops check <canister>) to verify the migration chain compiles
2. Run mops build to produce the wasm artifact

Agents consistently stop after mops check and consider the migration complete. The skill does have a detailed checklist further down that includes both steps, but agents that apply the migration pattern without reading the full checklist will miss the build step.

Proposed Addition

Add the following after "Create migration files directly in the chain directory.":

After mops check --fix (or mops check <canister>) confirms the chain compiles, run mops build to produce the wasm artifact. Do not skip mops build — mops check only type-checks and validates the chain; it does not produce output.

Context

Identified during eval runs for icskills PR dfinity/icskills#194 (upstream sync motoko 1.8.2 / mops cli-v2.13.2). The "Migration workflow" eval case consistently passed 4/5 — the failing behavior was specifically the mops build step after mops check --fix.