runPhaseStepSession was passing the full prompt string as both the
user-visible prompt: argument and as systemPrompt.append, sending
the same (potentially large) text twice per invocation and doubling
the token cost for every phase step session.
runPlanSession correctly uses a short directive as the user message
and reserves the full content for systemPrompt.append only. Apply
the same pattern to runPhaseStepSession: use a brief
"Execute this phase step: <step>" directive as the user message.
Closes#2194
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
1005f02db2