Re-add transfer guide

docs/transferring-work-from-eten-tech-foundation.md

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+# Transferring work from the eten-tech-foundation repository
+
+`paranext/scripture-editors` was seeded from
+[`eten-tech-foundation/scripture-editors`](https://github.com/eten-tech-foundation/scripture-editors)
+with a **curated** set of refs:
+
+- branches `main`, `platform-yalc`, and `release-prep`
+- all 80 release tags (`platform_v*`, `utilities_v*`, `scribe_v*`)
+
+Everything else — feature branches, work-in-progress branches, and any personal tags — was
+deliberately left behind. Nothing is lost: the other repository still has them, and because this
+repo carries the same commit history, git can move any of them across at any time.
+
+This guide covers moving that work over. It is all ordinary `git push`; none of it is destructive,
+and every step is reversible.
+
+## One-time setup
+
+Add the other repository as a remote alongside `origin`. Use `eten-tech-foundation` as the name —
+not `upstream`, which wrongly implies we track it.
+
+```bash
+git remote add eten-tech-foundation https://github.com/eten-tech-foundation/scripture-editors.git
+git fetch eten-tech-foundation
+```
+
+### If your existing clone still points at eten-tech-foundation
+
+You likely have a checkout whose `origin` is the old repository. Rather than overwriting that URL,
+keep it under its own name so nothing is lost and the change is trivially undone:
+
+```bash
+git remote rename origin eten-tech-foundation
+git remote add origin https://github.com/paranext/scripture-editors.git
+git fetch --all
+```
+
+`git remote -v` should now show both. To undo, reverse the rename.
+
+> paranext-core's `preinstall` checks this: if it finds a checkout whose `origin` is not the URL in
+> `dev-packages.json`, it stops and prints the exact commands rather than silently building from the
+> old remote.
+
+## Moving a branch that exists on eten-tech-foundation
+
+```bash
+git fetch eten-tech-foundation
+git push origin eten-tech-foundation/<branch>:refs/heads/<branch>
+```
+
+That copies the branch to this repo without needing a local checkout. Verify:
+
+```bash
+git ls-remote --heads origin <branch>
+```
+
+## Moving a branch that only exists locally
+
+```bash
+git push origin <branch>
+```
+
+Check first that it is based on something this repo has — a branch built on a commit that never
+reached `main` will bring that history with it, which is usually fine but worth knowing:
+
+```bash
+git log --oneline origin/main..<branch> | wc -l
+```
+
+## Moving several branches at once
+
+```bash
+git fetch eten-tech-foundation
+for branch in my-branch another-branch; do
+  git push origin "eten-tech-foundation/$branch:refs/heads/$branch"
+done
+```
+
+## Moving tags
+
+Release tags are already here. For a personal or backup tag:
+
+```bash
+git push origin <tag>
+```
+
+## Open pull requests
+
+A pull request cannot be moved between repositories. For each one still open against
+`eten-tech-foundation/scripture-editors` that should continue here:
+
+1. Push its branch to this repo using one of the recipes above.
+2. Open a new PR here with that branch.
+3. Close the old PR with a comment linking to the new one, so the trail is followable.
+
+Because this repository is **not** a GitHub fork, opening a PR from a branch here defaults its base
+to `paranext/scripture-editors`. That is the whole reason it was seeded as a standalone copy rather
+than with the Fork button — a fork always defaults new PRs to the parent repository, and GitHub
+offers no setting to change it.
+
+## Verifying a transfer
+
+```bash
+# Same tip commit in both places?
+git ls-remote --heads origin <branch>
+git ls-remote --heads eten-tech-foundation <branch>
+
+# Anything on the old branch that did not come across?
+git fetch --all
+git log --oneline origin/<branch>..eten-tech-foundation/<branch>
+```
+
+An empty third command means everything arrived.