docs(claude): diff3 fabricates a false-clean merge on the NUL-sentinel file

CLAUDE.md

@@ -1324,3 +1324,16 @@ acts.** This section is the fix; the write-up that noticed the problem was not.
   Same false-zero family as the 08-17 seven-zeros table: the miss is silent, the rc is clean, and
   the hidden file is exactly the CLI file most sweeps target. A fleet broadcast went out same
   night; this entry is the boot-loaded copy.
+- **ADDENDUM 2026-09-04 (murph) — the NUL sentinel also breaks GNU `diff3`, and worse than the
+  grep/rg misses above: it doesn't return a false zero, it FABRICATES a false-clean 3-way merge.**
+  Diagnosing a real conflict on `src/cli/bus.ts` (cortextos#151 vs main), extracted the three blobs
+  (base/ours/theirs) to plain files and ran `diff3 -m` — exit 0, zero conflict markers, read as "no
+  overlap, safe to auto-merge." The merged output had silently DROPPED the entire PR side's changes
+  (`listAgents` import, new `validateAssigneeArg` function, both call sites — verified via direct
+  byte-level python3 check, not grep, since the wrapper bug covers this file too). `git merge-file`
+  (git's actual merge algorithm, not GNU diff3) on the same three files correctly produced the real
+  conflict markers. **Apply: for any 3-way merge/conflict check on a file that might carry this NUL
+  sentinel (`bus.ts` today, any future NUL-carrying file), use `git merge-file` or `git merge-tree`,
+  never GNU `diff3` on extracted blobs — a silent false-zero is bad, a tool that hands back a
+  confident, mergeable-looking, WRONG result is worse, because there is no failed-lookup shape to
+  notice.**