fix(sdk): pass phaseDir to executor prompt so SUMMARY.md lands in .planning/

The SDK's buildExecutorPrompt told executors to "Create a SUMMARY.md file"
with no directory path, causing them to write it in cwd (project root)
instead of .planning/phases/{phase}/. Thread phaseDir from PhaseRunner
through PromptFactory and into the completion instructions so the executor
gets an explicit path like `.planning/phases/01-auth/01-01-SUMMARY.md`.

Backward compatible — buildExecutorPrompt still accepts a plain string
(agentDef) for existing callers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sdk/src/session-runner.ts

@@ -59,9 +59,10 @@ export async function runPlanSession(
   agentDef?: string,
   eventStream?: GSDEventStream,
   streamContext?: EventStreamContext,
+  phaseDir?: string,
 ): Promise<PlanResult> {
   // Build the executor prompt
-  const executorPrompt = buildExecutorPrompt(plan, agentDef);
+  const executorPrompt = buildExecutorPrompt(plan, { agentDef, phaseDir });
 
   // Resolve allowed tools — from agent definition or defaults
   const allowedTools = options?.allowedTools ??