Roll out nonempty-diff.yml to the standard repos #33

What

symphony (mark-brannan/symphony#69, #72) is the only repo using the reusable nonempty-diff.yml workflow from this repo. It should go to the other standard repos too: colregs, colregs-engine, searoom, dotfiles, nav-wright, wire-wright, ampacity, coastlines (and any other vended repo with a ci-gate ruleset).

Pattern

For each repo, follow the standard inheritance pattern documented in nonempty-diff.yml's own header — list it in the caller's ci-gate needs:, no ruleset entry required, since those repos' CI runs on pull_request (unlike symphony, which runs validate.yml on push and had to give nonempty-diff its own required ruleset context instead — see mark-brannan/symphony#69 for why that repo is the exception).

Scope

One PR per repo, each: add the nonempty job calling mark-brannan/.github/.github/workflows/nonempty-diff.yml@main, add it to ci-gate's needs:. No ruleset change needed if ci-gate / gate is already the sole required context.