test(symlinkguard): pin the alert for a special file Watch cannot remove (#179)

Review round 1 on PR #190:
- B1: dropping the unremovable special files from Watch's persistent
  finding left every test green. New test: a FIFO in a read-only dir
  stays and the alert names it; red under that flip.
- The removeSpecial doc and test comment no longer claim a regular file
  swapped in is always left alone: the re-check narrows, but cannot close,
  the window before the unlink (as in removeSymlinks).
- Messages: the special-file finding says what one is and to delete it;
  the persistent alert no longer talks only about symlinks.
- Rule 9: the Watch removal test is named for what it observes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ingestion/internal/symlinkguard/symlinkguard.go

@@ -66,7 +66,7 @@ func (r Result) String() string {
 		parts = append(parts, fmt.Sprintf("%d symlink(s): %s", len(r.Symlinks), q(r.Symlinks)))
 	}
 	if len(r.Special) > 0 {
-		parts = append(parts, fmt.Sprintf("%d special file(s): %s", len(r.Special), q(r.Special)))
+		parts = append(parts, fmt.Sprintf("%d special file(s) (FIFO, socket or device; delete them): %s", len(r.Special), q(r.Special)))
 	}
 	if len(r.Unreadable) > 0 {
 		parts = append(parts, fmt.Sprintf("%d unreadable dir(s): %s", len(r.Unreadable), q(r.Unreadable)))
@@ -207,8 +207,8 @@ func Watch(ctx context.Context, brainDir string, interval time.Duration, n Notif
 			reported = key
 			if key != "" {
 				slog.Error("brain symlink tripwire", "finding", key)
-				q.push("brain symlink tripwire (hyperguild#157): " + key +
-					". Readers may follow symlinks out of the brain; fix them.")
+				q.push("brain symlink tripwire (hyperguild#157, #179): " + key +
+					". Symlinks can lead readers out of the brain and special files can hang them; remove them.")
 			}
 		}
 	}
@@ -343,8 +343,10 @@ func permanent(err error) bool {
 
 // removeSpecial unlinks each brain-relative FIFO, socket or device through
 // an os.Root on the brain dir, after re-checking that it is still one: a
-// path that is now a regular file, a directory or a symlink is left alone
-// and reported as failed.
+// path that is a regular file, a directory or a symlink at the re-check is
+// left alone and reported as failed. The re-check narrows, but cannot close,
+// the window before the unlink (Linux has no conditional unlink); the same
+// window exists in removeSymlinks, and only a local writer can use it.
 func removeSpecial(brainDir string, rels []string) (removed, failed []string) {
 	if len(rels) == 0 {
 		return nil, nil