v1.21.0

Labels custom field support

cup field now supports the labels (multi-select) custom field type. Pass comma-separated label names:

cup field abc123 --set "Priority" "High, Medium"
cup field abc123 --set "Tags" "Backend"
cup field abc123 --remove "Priority"

Label names are resolved case-insensitively against the field's configured options. Invalid names show available options in the error message.

Supported field types: text, short_text, number, currency, phone, drop_down, labels, checkbox, date, url, email.

Fixes #47.

937 unit tests, 116 e2e tests.