Run one piece of work across several herdr agent sessions, one git worktree per unit.
Spread one piece of work across several herdr agent sessions — plan it with the operator, run each unit in its own git worktree with any configured agent and any saga capability, then merge the results back. Triggers on "/orchestrate", "orchestrate this", "split this across agents", "run these issues in parallel", "fan this out to multiple sessions", "orchestrate across vendors".
