docs(cache): recommend delete-on-merge, and say what ancestry still covers

`daniel/zemyna` enabled `default_delete_branch_after_merge` after this
branch was written, so the deleted-branch signal will fire there on
future merges. That makes the setting worth recommending — it is the
cheapest case for this scheme, decidable from `ls-remote` with no
checkout, no objects and no walk — and it does not make the ancestry
signal redundant.

Three things it leaves behind, now enumerated in the README's eviction
section rather than implied: every branch merged before the setting was
turned on, of which zemyna carried 48 and which nothing retroactively
deletes; every merge whose deletion the forge declines or is never asked
to make, since it is best-effort and silent and an API merge without the
flag never asks; and every repo that has not enabled it, which is the
default.

Two present-tense claims about one repo's configuration are reworded
into the conditions they were standing in for, in prune-cache.sh's
header and beside is_merged_dead, plus the two in the selftest that
asserted the forge keeps branches rather than describing the fixture.
No behaviour change; the suite is green and unchanged at 61 assertions.

Refs #20.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JXMQCJ5Eg5f9G9cfYzyh4Z

scripts/prune-cache.sh

@@ -28,8 +28,9 @@
 #      removed UNCONDITIONALLY, not gated on free space. A directory for a
 #      branch nothing will build again is pure loss; waiting for disk pressure
 #      to notice means paying for it until then. Two signals make a branch
-#      dead, and the second exists because the first alone is inert on a forge
-#      that keeps branches after merge (gitdan-actions#20):
+#      dead, and the second exists because the first alone is inert wherever
+#      a merged branch stays on origin — the default, and still the outcome
+#      whenever delete-on-merge declines or is not asked (gitdan-actions#20):
 #
 #        DELETED — the branch is no longer on origin at all.
 #        MERGED  — the branch is still on origin, but its tip is an ancestor
@@ -434,8 +435,11 @@ fi
 #
 # True when the branch this directory belongs to is still on origin but every
 # commit it holds is already on a protected branch — a merged PR whose branch
-# the forge did not delete, which is the case zemyna hits on every merge and
-# which the deleted-branch signal above can never see.
+# the forge did not delete, which the deleted-branch signal above can never
+# see. Enabling delete-on-merge narrows this to the branches merged before it
+# was enabled, the ones its deletion declines (protected, or used by another
+# open PR), and the merges that never ask (an API merge without the flag);
+# see README's eviction section.
 #
 # Memoised per tip because target-<key> and snapshot-<key> share one branch,
 # and because the "cannot tell" warning belongs to the branch rather than to