Empty markdown output prints '_(no records in window)_' instead of being empty — breaks 'data only on stdout' contract #16

The contract: "Empty = exit 0, empty output ([] for JSON)". The CLI honors this for JSON ([]) but for markdown emits a sentinel string on stdout:

// cmd/format.go
func emptyMsg(w io.Writer) error {
    _, err := fmt.Fprintln(w, \"_(no records in window)_\")
    return err
}

Repro:

$ /tmp/crono-export biometrics --since today --until today > /tmp/out 2>/tmp/err
$ cat /tmp/out
_(no records in window)_

This breaks scripts that pipe markdown output into something like pandoc or simply check [ -s file ] to decide whether to send the report. It's also a stdout-vs-stderr contract violation: the message is meta-information, not data, so it belongs on stderr (or simply omitted).

Expected behaviors (pick one, ideally the first):

1. Empty result -> empty stdout (zero bytes), exit 0. (Matches the JSON branch, which prints [] only because that is the data.)
2. If a human-readable hint is desired, print it on stderr, not stdout.

Affected callers: renderServings, renderBiometrics, renderExercises, renderNutrition, renderNotes all delegate to emptyMsg.

Severity: major