fix: emit [] for empty JSON; correct workouts category prime note (#11)

Two gaps the new 'prime' subcommand surfaced when its examples and the
output-formats contract were exercised end-to-end:

- Every subcommand's --format json was emitting 'null' on empty windows
  (the underlying slice was nil, which encoding/json marshals as null).
  The prime promises "empty result ... JSON prints '[]'", and 'null'
  also breaks the prime's sleep-efficiency jq pipeline. Special-case
  nil slices in printJSON via reflect so all five subcommands emit
  '[]' on empty.

- The workouts gotcha implied JSON carried both a string 'category' and
  a numeric 'category_code', but JSON only carries the integer
  'category' (the string mapping happens in markdown/CSV). Rewrite the
  gotcha to describe actual behavior and list the common code values
  inline so an LLM can filter by 'category == 16' without round-tripping
  through markdown.

Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>

Author: DTTerastar

cmd/prime.go

@@ -135,9 +135,11 @@ GOTCHAS
     (250ms between calls); ad-hoc loops should do the same.
   - Sleep score and apnea fields are populated only on supported devices.
     'data.sleep_score' is null on unsupported wakeup-light models.
-  - 'workouts' category codes are integers; common ones are mapped to
-    string names (walk/run/bicycling/...) but unknown codes pass through
-    as 'unknown' with the numeric category_code preserved.
+  - 'workouts' category is a Withings integer code in JSON (1=walk,
+    2=run, 6=bicycling, 16=lift_weights, ...).  Markdown and CSV map
+    common codes to string names ('lift_weights', 'walk', ...) and
+    unknown codes render as 'unknown'; CSV also keeps the raw integer
+    in a 'category_code' column.
 `
 
 var primeCmd = &cobra.Command{

cmd/shared.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
+	"reflect"
 	"strconv"
 	"strings"
 	"time"
@@ -108,6 +109,12 @@ func untilDayOrToday(s string) (time.Time, error) {
 }
 
 func printJSON(v any) error {
+	// A nil slice marshals as "null"; force "[]" so empty windows match the
+	// prime contract (and don't blow up jq pipelines).
+	if rv := reflect.ValueOf(v); rv.Kind() == reflect.Slice && rv.IsNil() {
+		_, err := os.Stdout.WriteString("[]\n")
+		return err
+	}
 	enc := json.NewEncoder(os.Stdout)
 	enc.SetIndent("", "  ")
 	if err := enc.Encode(v); err != nil {