The dashboard's estimate endpoints make the same headless auxiliary call
as specify/decompose but never bound an affinity scope, so they still sent
no x-opencode-session and the OpenCode Go relay answered 400
MissingSessionID (#112043). Declare kanban:<task_id> for an existing task
and a stable kanban:estimate key for the create dialog (no task yet),
unless a scope is already bound.
Test: _run_estimate captured header None before; now kanban:t_1 /
kanban:estimate and nothing leaks past the call.
f7ea39481a