Add MERGE-SOP.md

MERGE-SOP.md

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+# Merge SOP
+
+A pull request is merged when a maintainer who did not write it approves it.
+
+## Scope
+
+This rule applies to every repository in aska-digital, Foundation-0, and Proven-Commons.
+Maintainers are the members of the organisation that owns the repository.
+
+This SOP covers pull requests to main.
+
+## Rule
+
+Merge a pull request to main when all of these hold:
+
+- The pull request is open and is not a draft.
+- Its latest review from a maintainer is APPROVE.
+- The approving maintainer is not the pull request author.
+- The approval is on the current head.
+- The pull request merges cleanly into main.
+- No required check is failing.
+- No note in the pull request says an owner decision or a maintainer decision is still required.
+
+An author's own approval does not count. GitHub rejects an approval where the reviewer login matches the author login.
+
+## Merge step
+
+- Recheck the list above on the current head immediately before merging.
+- Merge with a merge commit, unless the repository uses a different method by default.
+- After merging, check that the pull request shows closed and merged.
+
+## Holds
+
+Do not merge when any item in the rule fails.
+
+- A dirty pull request needs a rebase first.
+- A failing check needs a fix or a recorded maintainer decision first.
+- A draft needs to be marked ready first.
+- An approval on an old head needs a fresh approval on the new head.
+- A pull request stacked on an unmerged branch waits until that branch lands on main.