i_will_plan now requires approach (min_length=20) at the schema and
non-empty sub_tasks at the gateway when the caller is a PM role. Restores
pre-gateway parity for _validate_claimed_start — agents could not
transition claimed -> in_progress without filling the rich plan.
Smoke run 3 (2026-05-11) showed PMs calling i_will_plan with just
plan='paragraph' and the gateway accepting it; Plan tab stayed empty
because no agent filled approach/sub_tasks/risks/open_questions.
Spec ref: docs/superpowers/specs/2026-05-12-post-smoke-3-fixes-design.md
section A1.
Renn F
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