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Merge remote-tracking branch 'origin/docs/worktree-guidance' into HEAD

Paul Campbell pushed by paul@keymail.dev 3339ff61a700029cc28418ea90c9f8f02ffea523 parent e9ccfe4parent 2e4e9c8
1 file changed, +38 −0
  • AGENTS.md +38 −0

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@@ -344,3 +344,41 @@ quietly ignored).
🤖 / 👨 markers, `Co-Authored-By: Claude …` trailers, published prompt and
carbon ledgers. AI is a guest with a name tag: opt-in, disclosed, and every
path works with it switched off.
+
+## Work in a worktree, and catch main up first
+
+Take a worktree for the work. Do not work in the shared checkout.
+
+ git -C <repo> fetch origin
+ git -C <repo> worktree add ../<repo>-<task> -b <branch> origin/main
+
+Both halves matter, for different reasons.
+
+**The worktree**, because several sessions share this machine and some
+share this repository, and a checkout has one HEAD, one index and one
+working tree between all of them. A branch switch under somebody else's
+uncommitted work destroys it with nothing on screen to say so, and two
+sessions editing the same paths disagree quietly instead of conflicting
+loudly. A worktree gives the work its own HEAD and its own files, and
+costs a directory.
+
+**From `origin/main`, after a fetch** — not from whatever the checkout
+happens to be sitting on. A branch cut from a stale main is a merge
+conflict scheduled for later, against files somebody else has renamed in
+the meantime. A branch cut from another task's branch is worse: it
+carries that task's commits into yours, where they are reviewed as
+though you wrote them, and the two can only be separated by hand.
+
+Remove it when the work has landed — `git worktree remove` — so that
+`git worktree list` keeps saying something true.
+
+Written down because of 2026-09-17, in `meet`. The shared checkout was
+sitting on one task's branch while holding a *different* task's feature
+uncommitted on disk — some four hundred lines, a new migration among
+them — put there by a session nobody else knew was working in that tree.
+Two other sessions were meanwhile coordinating over the same checkout by
+asking each other not to touch it, which is the arrangement a worktree
+makes unnecessary. Nothing was lost. Nothing about the arrangement would
+have said so if it had been, and by then the same feature existed twice,
+once on each side of a rename, because the two sessions had branched
+from different starting points.