aSPARK v0.3.1 — Propose the next feature

Adds /next-steps: when there's no idea in hand, the Product Owner surveys
the project's current state - shipped features from git history, in-flight
and stalled work from .spark/ artifact statuses (including open review/QA
findings), and the constitution's Product Principles - and proposes one
concrete next feature with a ranked alternative or two.

Advisory only: nothing is written to .spark/ and nothing needs approval.
The output hands straight into /story-time or /spark once the user picks
(or overrides with their own idea). /spark itself now points here instead
of guessing when there's no feature left to resume.