test(cli): a smoke case checks what it broke - #771

Four cases in `smoke-tools.sh` damaged a file and proved nothing about it.

They did not know which file they damaged. Four places picked one with
`find … | head -1`, and `find` answers in directory order — not sorted, not the
same on two filesystems — so each run corrupted whichever file came back first.

And the three restore cases never checked the repair: each appended drift, ran
`restore --force`, and asserted exit 0. A `restore --force` that exits 0 having
restored nothing is precisely the failure #762 fixed inside the command, so the
case meant to cover it would have passed throughout. The `ide` case was worse:
its drift was a bare newline, which `grep` cannot see, so no check on that file
was possible at all.

`first_file` (`LC_ALL=C sort | head -1`) replaces every unsorted pick. One named
marker, `SMOKE_DRIFT`, is what the three cases append, and `repaired` fails when
the mark is still there after the restore, or when nothing was drifted to repair.

The harness is not run by CI, so the three guards read its text, the way
`smoke-harness-isolation.unit.test.ts` already guards the home sandbox. Each was
written first and failed for the reason it names; `bash -n` and
`shellcheck -S error` are both clean.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01VWNxk63AGKkqE8HRqHLjGp