📖 [Docs]: Standardize separating reusable workflows from their action library

[Marius Storhaug (MariusStorhaug)]

What & why

There was no standard for how to split a reusable workflow from the actions
only it consumes. This documents it.

A shared reusable workflow loads only its own file into the caller's run — its
repository is not checked out — so a ./-relative action reference resolves
against the caller's checkout and breaks. A shared reusable workflow must
therefore reference actions by full owner/repo/path@<sha>, which dictates where
those actions have to live.

Changes

• Coding-Standards/GitHub-Actions.md — new section "Separate a reusable
  workflow from the actions it consumes" (right after "Choose an action or a
  reusable workflow"):
  • why a shared reusable workflow ships only its own file, and the contrast that
    a composite action's ./ resolves within its own repo at the same ref
    (the asymmetry that lets a set of actions ship together);
  • the two cross-repo homes — a standalone action repo (consumed on its own)
    vs an action-library repo (a cohesive set built, tested, and released
    together as one package), tied into the
    Repository Segmentation
    shared-lifecycle principle;
  • a pointer bullet added to "Start local; promote when it is reused."

Companion

Companion to AI-Platform/ai-platform#411, which adds the same standard to the
mirrored coding standards (and a matching rule in that repo's
repository-segmentation.md). MSXOrg's Repository-Segmentation.md already
carries the shared-lifecycle principle, so only the coding standard changes here.

Validation

• markdownlint (repo config) clean.
• New section and all anchor targets (in-file and the cross-file segmentation
  anchor) verified to resolve.