A megamerge combines several parallel branches into one working commit.
Moving the working copy off it discards that combined view and forces a
manual re-merge to reconstruct it, so document that agents should stay
put unless explicitly asked to move.

Assisted-by: Claude (Opus 4.8)

diff --git a/ai/_AGENTS.md b/ai/_AGENTS.md
index b1993895..52f876eb 100644
--- a/ai/_AGENTS.md
+++ b/ai/_AGENTS.md
@@ -100,6 +100,13 @@ Required skills for jj operations:
 
 Use `jj commit` to commit the current change, not `jj describe`.
 
+Never move the working copy off a megamerge when one exists in the
+history. A megamerge is a single working commit that integrates several
+parallel branches at once; moving off it (via `jj new` or `jj edit` onto
+another commit) discards that combined view, and rebuilding it means
+re-running the merge by hand. If a megamerge is present, stay on it
+unless I explicitly ask you to move.
+
 Commit quality:
 - Each commit should be one logical unit of change
 - Every commit must build and pass checks (bisect-able history)