craftkit v1.56.0

Execution units: pick the smallest one that fits

Agents had rules for when to spawn (parallel orchestrators, fusion panel) but none for which
unit to use, so coupled work got split across subagents and parallel edits could share a checkout.

• using-agent-skills gains core behavior #12: main task, subagent, custom agent, worktree, or
  fork, each with its use. Delegation follows one flow: workstream contract, permission boundary,
  read-only subagent or isolated worktree, main-task review, human decision.
• The Codex runtime block carries a four-line version, since Codex loads only that section.