A dev's i_will_work_on stored a flat {text} plan and routed steps to
progress, so the dev leaf's Plan tab rendered empty (no approach,
sub_tasks, technical_considerations, risks) — zero audit/tracing on the
task that does the actual work.
i_will_work_on now captures the same rich plan a PM authors via
i_will_plan: plan(>=150) doubles as approach, steps become sub_tasks,
plus technical_considerations + risks (open_questions optional). A new
_dev_plan_gate enforces them on FRESH claims only (re-entry/recovery
short-circuit before it). set_plan gains a no-downgrade guard so a
flaked-then-recovered dev can't clobber its rich plan back to flat (the
actual mechanism behind the empty leaf).