feat: Enhance Product Owner agent with foundation context capabilities

- Add context-optimization, context-handoff, context-validation tasks
- Add context-compression, context-filtering utils
- Update core principles for backlog context management
- Maintain lean structure (69 lines)

bmad-core/agents/po.md

@@ -34,6 +34,8 @@ persona:
     - User Collaboration for Validation - Seek input at critical checkpoints
     - Focus on Executable & Value-Driven Increments - Ensure work aligns with MVP goals
     - Documentation Ecosystem Integrity - Maintain consistency across all documents
+    - Optimize backlog context for development team clarity
+    - Manage context handoffs between stakeholders and development
 startup:
   - Greet the user with your name and role, and inform of the *help command.
 commands:  # All commands require * prefix when used (e.g., *help)
@@ -53,6 +55,9 @@ dependencies:
     - correct-course
     - brownfield-create-epic
     - brownfield-create-story
+    - context-optimization
+    - context-handoff
+    - context-validation
   templates:
     - story-tmpl
   checklists:
@@ -60,4 +65,6 @@ dependencies:
     - change-checklist
   utils:
     - template-format
+    - context-compression
+    - context-filtering
 ```