Leave removed copies out of report and trees (closes #173)

A deleted duplicate stayed in both reports until a scan covered it, and
a missing scan operand is a fatal error. report now runs lstat on each
path of a possible duplicate group and drops paths that are gone; trees
does the same for the directory at the top of each copy, never the
files inside. A group left with one copy is not reported. Only "does not
exist" and "not a directory" count as gone, so a permission error keeps
the copy for an unprivileged user. README says a stale report is fixed
by scanning again, and its Docker example mounts the scanned tree for
report too.

Model: opus-5-5

TODO.md

@@ -29,6 +29,11 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `report` and `trees` leave out a copy that was deleted after the scan. Each
+  checks only the copy itself with `lstat`: the file, or the directory at the
+  top of a tree. A group left with one copy is not reported. A missing `scan`
+  operand is still a fatal error (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/173)
 - The usage line of `scan` no longer ends in `[flags]`. It is the line
   `README.md` shows under "Subcommands" (2026-11-11,
   https://git.eeqj.de/sneak/sfdupes/issues/169)