story-kit: create 308_bug_token_cost_breakdown_missing_from_expanded_work_item_detail_panel

.story_kit/work/1_backlog/308_bug_token_cost_breakdown_missing_from_expanded_work_item_detail_panel.md

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+---
+name: "Token cost breakdown missing from expanded work item detail panel"
+---
+
+# Bug 308: Token cost breakdown missing from expanded work item detail panel
+
+## Description
+
+Story 300 implemented the token cost badge on the pipeline board but skipped the acceptance criterion for showing per-agent cost breakdown in the expanded work item detail panel. When you click into a story, there's no cost information visible.
+
+## How to Reproduce
+
+1. Click on a work item that has token usage data (cost badge visible on board)
+2. Look at the expanded detail panel
+3. No cost breakdown is shown
+
+## Actual Result
+
+Expanded work item detail panel shows no token cost information.
+
+## Expected Result
+
+Detail panel shows per-agent cost breakdown (coder, QA, mergemaster sessions) with token counts and cost in USD for the selected story.
+
+## Acceptance Criteria
+
+- [ ] WorkItemDetailPanel fetches token cost data using the existing /work-items/:story_id/token-cost endpoint
+- [ ] Shows per-agent session breakdown: agent name, model, token counts (input/output/cache), cost
+- [ ] Shows total cost for the story
+- [ ] Shows empty state when no token data exists