Phase 1 now writes OutlinePath to a ClickUp custom field via
set_custom_field_by_name(). Phase 2 reads it back with
get_custom_field_by_name(), falling back to convention path
({outline_dir}/{slug}/outline.md) if the field is empty.
Added get_task(), set_custom_field_by_name(), and
get_custom_field_by_name() helpers to ClickUpClient.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>