fix: render unset timestamps as null in --json output

go-flashduty's Timestamp.MarshalJSON emits a quoted RFC3339 string for a
set value but the bare integer 0 for the unset sentinel, so one field
switches JSON type depending on record state: an active alert's end_time
is the number 0 while a recovered alert's is a string. jq arithmetic over
mixed-state output fails with "string and number cannot be subtracted".

Add output.NullUnsetInstants, a reflection-based transform applied at the
JSON marshaling boundary (JSONPrinter.Print and marshalStructured), that
replaces every unset SDK timestamp with nil so it renders as null. Set
values keep their concrete type and custom MarshalJSON, non-timestamp
zero integers are untouched, and struct field order is preserved
(verified byte-identical when no unset instant is present). TOON and
table output are unchanged: they render the sentinel through String(),
already always a string.

The generated help note and the skill card prose that documented the old
"unset stays the bare integer 0" contract are updated to match.

Author: ysyneu

internal/cli/root.go

@@ -258,13 +258,14 @@ func currentOutputFormat() output.Format {
 }
 
 // marshalStructured serializes v for machine-readable output: indented JSON for
-// FormatJSON (byte-compatible with the legacy --json path) and TOON via the
-// toon-format encoder for FormatTOON.
+// FormatJSON (byte-compatible with the legacy --json path, except that unset
+// SDK timestamps now render as null instead of the bare integer 0 — see
+// output.NullUnsetInstants) and TOON via the toon-format encoder for FormatTOON.
 func marshalStructured(v any) ([]byte, error) {
 	if currentOutputFormat() == output.FormatTOON {
 		return toon.Marshal(v)
 	}
-	return json.MarshalIndent(v, "", "  ")
+	return json.MarshalIndent(output.NullUnsetInstants(v), "", "  ")
 }
 
 // newPrinter creates a Printer based on global flags.