fix(frontend): split pending board idle state

[laxmanclo]

Summary

• rename the board's first work column to Pending
• split that column into a simple Working section and an Idle section
• collapse Idle down to a quiet count row when there are no idle sessions
• add a focused board test for the working/idle grouping

Testing

• npm --prefix frontend test -- src/renderer/types/workspace.test.ts src/renderer/components/SessionsBoard.test.tsx

Note

• Full frontend typecheck still hits the existing upstream Forge typing issue at forge.config.ts:33 (osxNotarize).

[laxmanclo]

Closing because this change belongs in AgentWrapper/agent-orchestrator, not this repo.