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

cargo-cache/action.yml

@@ -24,7 +24,9 @@ inputs:
     default: ''
   protected-branches:
     description: >-
-      Space-separated refs that publish snapshots and are never evicted.
+      Space-separated refs that publish snapshots. A protected ref's
+      snapshot is never evicted; its own target dir is an ordinary
+      pressure-pass candidate, reseeded from the snapshot on its next run.
       These are the branches PR caches layer over.
     required: false
     default: 'dev main'