feat(tui): filter subagents by activity

[thdxr]

Summary

• show only active sub-agents when opening the TUI composer picker
• use Tab to toggle between active and inactive sub-agents
• show the available toggle action in the picker header

Testing

• bun test --timeout 30000 --only-failures in packages/tui (507 pass, 5 skip)
• bun typecheck in packages/tui
• bun run lint packages/tui/src/mini/footer.command.tsx packages/tui/test/mini/footer.view.test.tsx (0 errors; existing warnings)

Notes

• The repository-wide pre-push typecheck fails in unrelated packages/app code because @opencode-ai/client and @opencode-ai/client/promise declarations are unavailable and compatibility types are stale on this branch.