fix(sdk): stop duplicating prompt in runPhaseStepSession user message (#2223)

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>

sdk/src/session-runner.ts

@@ -277,7 +277,7 @@ export async function runPhaseStepSession(
   const cwd = options?.cwd ?? process.cwd();
 
   const queryStream = query({
-    prompt: prompt,
+    prompt: `Execute this phase step: ${phaseStep}`,
     options: {
       systemPrompt: {
         type: 'preset',