Test the remaining CLI cases (closes #16)

Most of the command-line contract was already tested through run. This
adds what was missing: report and trees with no database exit 1 with
the message telling the user to run scan; a scan that skips an
unreadable file prints its warning and counts the skip in its summary;
the report and trees summary lines are checked exactly. The scan tests
now capture the process's own stdout, which scan would write to
directly, so a stray stdout write in scan fails them. The fatal-path
test takes its subcommands from the command tree, so a new subcommand
wired without runE fails it. The nonexistent-operand test gets an
accurate name.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- tests cover a missing database, `scan` keeping stdout empty, its skip
+  warning, the `report` and `trees` summary lines, and every subcommand
+  going through `runE` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/16)
+
 - `.golangci.yml` replaced with the current canonical copy, which uses
   `gomodguard_v2`, so lint no longer prints a deprecation warning
   (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/26)