Fix leaked source paths in PRD validation report (bmad-code-org#1481)

Replace /src/core/ paths with {project-root}/_bmad/core/ convention
in validation-report-prd-workflow.md lines 293-294.

These paths reference party-mode and advanced-elicitation workflows
using the source tree layout (/src/core/) instead of the installed
layout ({project-root}/_bmad/core/) that Beta.4 standardized
across all other files.

This is the remaining half of the fix for bmad-code-org#1435, which was closed
as "will fix with .4 release" but only the brianmadison path on
line 11 and the XML syntax issue were addressed. The /src/core/
paths on lines 293-294 were missed.

Fixes bmad-code-org#1480

Author: arcaven

src/bmm/workflows/2-plan-workflows/create-prd/validation-report-prd-workflow.md

@@ -290,8 +290,8 @@ editWorkflow: './steps-e/step-e-01-discovery.md'
 - ✅ Proper workflow path configuration
 
 **2. External Workflow References:**
-- ✅ Party-mode workflow: Exists at `/src/core/workflows/party-mode/workflow.md`
-- ✅ Advanced-elicitation task: Exists at `/src/core/workflows/advanced-elicitation/workflow.xml`
+- ✅ Party-mode workflow: Exists at `{project-root}/_bmad/core/workflows/party-mode/workflow.md`
+- ✅ Advanced-elicitation task: Exists at `{project-root}/_bmad/core/workflows/advanced-elicitation/workflow.xml`
 
 **3. Directory Structure:**
 - ✅ Complete step architecture (all 3 modes)