fix(tui): improve subagent picker states

[opencode-agent]

Summary

• keep Ctrl-B backgrounding active inside the V2 subagent picker
• distinguish foreground running subagents with the standard spinner and backgrounded subagents with a label
• dim the agent type and remove the colon

Validation

• bun typecheck from packages/tui
• OpenCode Drive scripted foreground → Ctrl-B → background flow
• recorded and inspected a 1120×680 MP4 demo