field-handler: run a room of field agents

Before you use this skill, check that HERDR_ENV=1. If it is not 1, tell the user that you do not run inside a herdr pane. Then stop.

Run herdr --skill for the command surface, then read the herdr skill for the semantics it leaves out and the field-agent skill for the dispatch contract; this is its multi-agent form, on the same ledger, watch loop and three rules.

Two directories matter below: