fix(prune): stop protecting publisher target dirs under disk pressure

A publisher branch's target-<ref> was protected identically to its
snapshot-<ref>, so it was never a pressure-pass candidate however old
and however tight the disk. With one branch's target dir permanently
resident alongside its snapshot, a second branch had no room to seed,
and every PR that night needed a hand eviction between runs
(daniel/gitdan-actions#24).

A publisher's target dir is a convenience cache its own next run
reseeds from the snapshot, so losing it under pressure is cheap;
nothing downstream depends on it surviving. Only the snapshot stays
protected in the pressure and self-clear passes.

Unprotecting the target dir outright surfaced a second bug the fix
would otherwise have shipped: a branch's tip is trivially an ancestor
of itself, so once a protected ref's target dir was no longer skipped
before reaching the merged-branch check, pass 1 read it as "merged
into itself" and deleted it unconditionally on every run, independent
of disk pressure. is_protected_from_liveness keeps a protected ref's
target dir out of pass 1 alone, so it stays an ordinary pressure-pass
candidate without ever reaching that check. Scenario 3b in the
selftest red-proves this against the unprotect-only version of the
fix.

Also corrects the header's inode-sharing claim, measured false on the
live volume by daniel/zemyna#1073: publish-snapshot.sh unshares every
executable after its cp -al, and executables are most of the tree by
bytes, so a snapshot eviction is a real, large disk cost rather than
the near-free one the old text described — the target-before-snapshot
ordering still holds, now for the warm-start reason alone plus that
cost.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UkSjXXtU6JYcN2vPntWhfb

scripts/prune-cache-selftest.sh

@@ -11,8 +11,13 @@
 #      Waiting for pressure to notice means paying for dead caches until then.
 #   2. LIVE BRANCH SURVIVES despite being OLDER than the dead one — liveness,
 #      not age, is what decides pass 1.
-#   3. PROTECTED REFS NEVER EVICTED under forced disk pressure, even when
-#      their caches are the oldest on disk and would rank first for LRU.
+#   3. A PROTECTED REF'S SNAPSHOT NEVER EVICTED under forced disk pressure,
+#      even when it is the oldest on disk and would rank first for LRU — its
+#      own TARGET dir is an ordinary candidate and goes (gitdan-actions#24).
+#  3b. AND A PROTECTED REF'S TARGET DIR SURVIVES PASS 1 ANYWAY: its tip is
+#      trivially an ancestor of itself, so the merged-branch signal must never
+#      be allowed to evaluate it, or pass 1 — unconditional, not gated on
+#      pressure — would delete it every run.
 #   4. LOCKED CACHE PROTECTED even when dead, old, and under pressure — and
 #      the pass's closing summary agrees with the decline it just logged,
 #      rather than reporting that it found nothing.
@@ -139,13 +144,34 @@ assert_kept "$root/target-$LIVE" "live branch survives despite an older marker t
 assert_log "no matching branch on origin" "eviction reason reported"
 
 echo
-echo "=== 3: protected refs never evicted under forced pressure ==="
+echo "=== 3: protected refs' SNAPSHOTS never evicted under forced pressure ==="
 reset_cache
 run_prune "1000000 1000"     # 0.1% free
-assert_kept "$root/target-$DEV" "dev's target dir survives disk pressure"
 assert_kept "$root/snapshot-$DEV" "dev's snapshot survives disk pressure"
-assert_kept "$root/target-$MAIN" "main's target dir survives disk pressure"
 assert_kept "$root/snapshot-$MAIN" "main's snapshot survives disk pressure"
+# Their TARGET dirs are ordinary candidates and go under the same pressure —
+# gitdan-actions#24: protecting them starved every second branch of room to
+# seed. Asserted here (gone, not kept) so this scenario still red-proves the
+# snapshot half if a future change reintroduces target protection.
+assert_gone "$root/target-$DEV" "dev's target dir is an ordinary pressure-pass candidate"
+assert_gone "$root/target-$MAIN" "main's target dir is an ordinary pressure-pass candidate"
+
+echo
+echo "=== 3b: a protected ref's target dir is NOT pruned by pass 1's liveness ==="
+# The regression this fix could introduce and the selftest above cannot see:
+# a protected branch's tip is trivially an ancestor of itself, so once its
+# target dir stopped being excluded from pass 1 altogether, is_merged_dead
+# read it as "merged into itself" and pass 1 — unconditional, not gated on
+# pressure — deleted it on every single run. Plenty of free space, so only
+# pass 1 can be responsible for anything gone here.
+reset_cache
+run_prune "1000000 900000"   # 90% free: no pressure at all
+assert_kept "$root/target-$DEV" "dev's target dir survives pass 1 despite being its own ancestor"
+assert_kept "$root/target-$MAIN" "and so does main's"
+if grep -q "merged into" "$scratch/log"; then
+  fail "a protected ref's own target dir was evaluated by the merged-branch signal at all"
+fi
+ok "no protected ref's own target dir reaches the merged-branch check"
 
 echo
 echo "=== 9: own cache never evicted by a sibling pass ==="