Import josh-sync from subrepo/main

Sync-Origin: import/josh-sync/20260528-041502

docs/adr/013-non-destructive-adoption.md

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+# ADR-013: Non-destructive adoption merge for existing subrepos
+
+**Status:** Accepted
+**Date:** 2026-04
+**Update (2026-05):** The standalone `josh-sync adopt <target>` command described below has been folded into `josh-sync onboard <target>` as the `adopt` strategy (selectable via `--mode=adopt`, `targets[].history_lock: preserve`, or auto-detection when the subrepo has any branches). `josh-sync adopt` is retained as a thin CLI alias. The state file moved from `<target>/adopt.json` to `<target>/onboard.json` with a `.strategy` field; the legacy file is still read for backward compatibility. See the guide's Onboarding section.
+
+## Context
+
+Existing subrepos often already have real history, open PRs, and developer clones.
+The original `import` then `reset` onboarding path establishes Josh-compatible
+history by force-pushing the Josh-filtered monorepo history onto the subrepo.
+That is correct for empty replacement repos, but it rewrites the active subrepo
+branch and invalidates local clones.
+
+We need a path for active subrepos where history must remain available on the
+same branch and where open PR branches should stay based on the existing history.
+
+## Decision
+
+Add `josh-sync adopt <target>` as a separate workflow from `onboard` and `reset`.
+Adoption imports the subrepo content into the monorepo, waits for the import PR
+to merge, then creates one merge commit on each configured subrepo branch:
+
+1. First parent: Josh-filtered monorepo HEAD
+2. Second parent: existing subrepo HEAD
+3. Tree: Josh-filtered monorepo tree
+
+Josh follows first-parent history back to the monorepo, while Git still keeps
+the old subrepo history reachable through parent 2. The push is a normal
+fast-forward push from the existing subrepo HEAD to the adoption merge commit.
+No force-push is used.
+
+Before creating the adoption merge, josh-sync requires the existing subrepo tree
+to match the Josh-filtered monorepo tree. If the trees differ, adoption aborts
+and the user must merge the import PR or reconcile the branch contents first.
+
+Adoption state is stored separately at `<target>/adopt.json` on the
+`josh-sync-state` branch.
+
+## Consequences
+
+**Positive:**
+
+- Existing subrepo history remains on the active branch.
+- Existing clones can fast-forward instead of hard-resetting or re-cloning.
+- Open PR branches remain based on reachable history.
+- Josh-compatible ancestry is established without a destructive rewrite.
+
+**Negative:**
+
+- Adoption adds one synthetic merge commit to each adopted subrepo branch.
+- Strict "no subrepo commit at all" adoption is impossible if the existing
+  subrepo branch must become connected to the Josh-filtered history without a
+  rewrite. A commit is needed to join the two histories.
+- Tree equality is strict. If the monorepo import differs from the subrepo
+  content, adoption stops until the user resolves the mismatch.