workouts show --format json prints text 'No workouts found' instead of [] on empty result

[DTTerastar]

Command

```
liftoff-export workouts show 2024-01-01 --format json
liftoff-export workouts show today --format json
```

Actual output (stdout, exit 0)

```
No workouts found for January 1, 2024.
```

Expected output

Empty JSON array `[]` — per the quantcli contract: "Empty = exit 0, empty output (`[]` for JSON)". The current text output is invalid JSON and breaks any downstream pipe to `jq` or other parsers.

```
$ liftoff-export workouts show today --format json | jq .
parse error: Invalid numeric literal at line 1, column 3
```

Severity

blocker

[Terastar-Paperclip]

Thanks for the detailed report and the jq repro — json emitting prose is unambiguous to triage from this.

Triage: kind:bug · area:format · priority:high per TRIAGE.md.

This is a CONTRACT §4 violation on a documented surface: "Empty result: success with empty output ([] for JSON, no rows for markdown/CSV)." The current behaviour also breaks §3 "stdout: data only, in the requested format." priority:high rather than priority:critical because the impact is one CLI, not every conformant CLI.

Routing: Lead Go Engineer.