Standardize HDDL core feature set and design choices

Determine and standardize the core feature set and associated design choices for the HDDL hierarchical planning language, specifying which features belong in the core language versus optional extensions to establish a community-agreed minimal standard.

Background

HDDL is proposed as a common, PDDL-aligned language for hierarchical (HTN) planning intended to unify disparate system inputs and foster comparability and adoption. The authors intentionally restrict the initial proposal to a basic feature set shared by many recent systems.

In the discussion, they explicitly note that finalizing this basic feature set and resolving design options requires community consensus, indicating that the scope and composition of the core HDDL language remain unresolved.