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-selftest.sh

@@ -46,10 +46,10 @@
 #      empty a tree its owner may still restore under a live cache name. Its
 #      fixture is an OLD directory renamed a moment ago — production's shape,
 #      and what lets it tell the two timestamps apart.
-#  15. A MERGED-BUT-UNDELETED BRANCH IS DEAD TOO. This forge keeps branches
-#      after merge, so `ls-remote` reports them forever and scenario 1's
-#      signal never fires for them — which is how three 40 GB caches sat on a
-#      full volume until somebody removed them by hand (gitdan-actions#20). A
+#  15. A MERGED-BUT-UNDELETED BRANCH IS DEAD TOO. A branch the forge did not
+#      delete at merge stays on `ls-remote` forever, so scenario 1's signal
+#      never fires for it — which is how three 40 GB caches sat on a full
+#      volume until somebody removed them by hand (gitdan-actions#20). A
 #      branch whose tip is an ancestor of a protected branch's tip is pruned
 #      like a deleted one; an unmerged branch beside it is not.
 #  16. AND "CANNOT TELL" IS STILL NOT DEATH, at both granularities: a branch
@@ -314,7 +314,7 @@ assert_log "may still be evicting it" "the deferral gives its actual reason"
 
 echo
 echo "=== 15: a merged-but-undeleted branch is dead too ==="
-# This forge keeps a PR's branch after the merge, so `ls-remote` reports it
+# A branch the forge did not delete at merge, which `ls-remote` then reports
 # forever. Built the way that happens: a branch merged into dev with a merge
 # commit, still pushed, beside one branched at the same point and NOT merged.
 git="git -c user.email=t@t -c user.name=t -c commit.gpgsign=false"