agents: the ancestor check prints nothing on success -- add the echo

sylpheed-port found this in their own §6 and it is in mine identically. I stated the
pass condition as 'exit 0' but left the command silent, so a reader running it as
written sees an empty line and cannot tell success from failure. Worse than a number
without a pass condition, because there is no number.

Now carries '; echo $?' and says why.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wuu56cE8vJGTBtn1ppsk8v

docs/agents/MERGE-STATE.md

@@ -22,7 +22,11 @@ dry-run merge is clean. Re-derive with:
 A=<decoder branch>; B=<port branch>
 
 git rev-list --count origin/main..$A        # rises with every commit; not the claim
-git merge-base --is-ancestor origin/main $A # PASS = exit 0 (fast-forward)
+git merge-base --is-ancestor origin/main $A; echo $?
+                                            # PASS = 0 (fast-forward)
+                                            # ⚠️ prints NOTHING on success without
+                                            #    the echo -- an empty line does not
+                                            #    distinguish pass from fail
 comm -12 <(git diff --name-only $(git merge-base $A $B)..$A | sort) \
          <(git diff --name-only $(git merge-base $A $B)..$B | sort) | wc -l
                                             # PASS = 0 (disjoint change sets)