chore(merge-driver): say what the scratch merge measured about the guide's removed route

A merge made from a checkout carrying the new .gitattributes conflicts on the
stub. A branch forked before this change still defers the guide on its first
merge of main, because git reads routes from the merging checkout; the merged
tree has no row to discharge the deferral against, so the next commit and the
push are refused where, with the route, they passed and the stub was lost.

Claude-Session: https://claude.ai/code/session_01VZqqwTj2wsihZEbfT6yyYN
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

.gitattributes

@@ -38,10 +38,15 @@
 # it any more. It is a hand-written pointer stub naming the guide's public address,
 # and `gen:upgrade-guide` writes the gitignored docs pages under
 # content/docs/protocol-upgrade/ instead (NOT_DRIVER_MANAGED records that path). A
-# route there would now be harmful, not idle: the driver keeps one side WHOLE, so a
-# merge between the stub and a side still carrying a generated guide would keep
-# whichever side the driver chose, with nothing left to regenerate it from. Unrouted,
-# that merge text-merges and conflicts on the stub.
+# route there was harmful, not idle: the driver kept OURS whole, and the pre-commit
+# discharge then passed it, because `check:upgrade-guide` compares no committed
+# copy — so a branch carrying a generated guide merged the stub away in silence.
+# Unrouted, a merge made from a checkout that carries this file conflicts on the
+# stub. ⚠️ git reads the route from the checkout DOING the merge, so a branch
+# forked before #22483 still defers the guide on its first merge of main; the merged
+# tree has no row to discharge it against, so the next commit and the push are
+# refused instead of passed (measured on PR #22556 with the previous head as the
+# control, both merge directions).
 #
 # `merge=os-regen` hands those paths to `scripts/git-merge-regen.mjs`, which does
 # NOT text-merge them. See that file for why it also does not regenerate them

scripts/regen-artifacts.mjs

@@ -932,9 +932,10 @@ export const NOT_DRIVER_MANAGED = Object.freeze([
       + 'runs before `next build`. Git never merges them. Recorded because this generator used to own '
       + 'a ROUTED row, `docs/protocol-upgrade-guide.md`, and #22483 took that row out on purpose: the '
       + 'file is now a hand-written pointer stub that nothing generates, so "discard both sides and '
-      + 're-run the generator" would discard the stub with no generator to bring it back. A merge '
-      + 'that meets it on both sides is a human\'s, and its conflict is the signal. Same expiry clause '
-      + 'as the three untracked entries above: committing these pages turns this entry red.',
+      + 're-run the generator" would discard the stub with no generator to bring it back, and '
+      + '`check:upgrade-guide`, which compares no committed copy, would pass the loss. A merge that '
+      + 'meets the stub on both sides is a human\'s. Same expiry clause as the three untracked '
+      + 'entries above: committing these pages turns this entry red.',
   },
   {
     path: 'docs/audits/**',