Guide hierarchical group setup and remove group stdin creation

[maelcaldas]

Summary

This PR simplifies early-stage group setup by removing the stdin batch input path from bkper group create and standardizing the recommended flow around explicit hierarchical creation.

Changes

• remove group create stdin batch creation and its unused implementation/tests
• document explicit book setup guidance in the README:
  • create top-level groups first
  • create child groups with --parent
  • create accounts with --groups
  • verify hierarchy and memberships before reporting success
• add README compliance rules/tests so the removed group-stdin flow is not documented again

Why

Groups define structure. Creating them explicitly keeps hierarchy setup predictable during book initialization and reinforces the intended setup order.

Although this removes an existing input path, this command surface is still in an early stage and the removed stdin flow has no meaningful adoption, so this is being released as a minor simplification rather than treated as a major compatibility event.

Verification

• bun run build
• bun run test:unit