fix(team-mode): refactor layout to use testable spawn-process helper

The existing layout.test.ts relied on mock.module("bun", ...) registered
at the top level, but test-setup.ts calls mock.restore() + restoreModuleMocks()
in afterEach, so every test except the first one lost its mocks. CI has
been red on this file since e303feef.

Two changes:

1. layout.ts now imports spawn from the existing spawn-process helper
   instead of "bun" directly, matching the pattern established for
   closeTmuxPane and killTmuxSessionIfExists. This does not change
   runtime behavior - spawn-process just re-exports Bun's spawn.

2. layout.test.ts registers module mocks inside beforeEach and uses the
   ?test=UUID cache-busting dynamic-import pattern so the mocks apply
   on every test run, not just the first.

All 4 layout.test.ts cases now pass.