Test the Atomic builtin stream with whichever builtin the stub calls (#306)

dev/integration carries #270, which withholds bash from the Planner's own
turns, so its stub reads a file instead. Pick the tool from the stream's
dynamic parts rather than naming bash, so the same tests hold on main and
on top of #270.

Assistant-model: Claude Opus 5.5
Assistant-workflow: inline
Assistant-verification: bun test passed: apps/server/src/harness/atomic/full.test.ts (18 pass) on main's base
Co-authored-by: Alex Lavaee <lavaman131@github.com>