Herdr child agents — a long prompt can fail to submit and look exactly like an idle child

A herdr agent prompt carrying a long brief (~150+ lines) can be delivered to an opencode child's TUI as a bracketed paste that is never submitted. The composer sits there showing [Pasted ~158 lines], the agent stays idle, and agent prompt still returns {"type":"agent_prompted"} with no error.

Short prompts of the same shape submit normally. Observed 2026-09-28 managing docker-images worktree children.

The failure is silent because idle is ambiguous: it is the state of a child that was never addressed and of one that is thinking, or done, or waiting its turn. A manager's notes saying "Dispatched" plus a live worktree plus an idle agent all look identical to a healthy run.

Why: a whole run can be lost without a single error. In that run the entire tick reduced to one changed line, so the loss was total and the checklist still read "Dispatched" with every box unticked.

How to apply:

• Put the brief in a file in the run directory (brief-<child>.md next to queue.md); prompt with one or two sentences plus the absolute path. Short prompts don't stall, and a file brief makes a re-prompt one line.
• After prompting, confirm the lifecycle actually moved: herdr agent get <name> must show working and a bumped revision within seconds. Unchanged revision means the prompt did not land.
• herdr pane read <pane> --source visible shows an unsent paste immediately.
• Clear a stuck composer with herdr agent send-keys <agent> ctrl+u before re-prompting, so the next submit does not concatenate with the dead paste.
• Related: a server restart can kill a child's shell tool mid-turn. Its staged work survives — check git status in its worktree before assuming a loss, then re-prompt; staged work only needs git commit and a push.