feat(agent): Enrich breakdown/decompose agent input with full initiative context

Breakdown and decompose agents now receive all existing phases, tasks,
and pages as read-only context so they can plan with awareness of what
already exists instead of operating in a vacuum.

src/agent/prompts/decompose.ts

@@ -30,6 +30,12 @@ ${ID_GENERATION}
 - Use \`checkpoint:*\` types for tasks requiring human review
 - Dependencies should be minimal and explicit
 
+## Existing Context
+- Read context files to see sibling phases and their tasks
+- Your target is \`phase.md\` — only create tasks for THIS phase
+- Pages contain requirements and specifications — reference them for task descriptions
+- Avoid duplicating work that is already covered by other phases or their tasks
+
 ## Rules
 - Break work into 3-8 tasks per phase
 - Order tasks logically (foundational work first)