Subagents

When collaboration tools are enabled, the harness can spawn subagents with their own session branches, depth limits, and a shared registry.

• Subagents — delegated agents with their own context (different repos, isolated research).
• Workers — parallel instances of the same agent on one project, coordinating through the store (path claiming, messages).

Parent session tracks the spawn graph in PROJECT/.elph/store.db. Child sessions get artifact dirs under APP_DATA/sessions/<child-id>/. Subagents inherit model and system-prompt config from the parent spawn config.