Delegate envelope: the required `prompt` is omitted on the first hand-off call, costing a wasted step

What happens

On a live account run (core built from #6304 at 2abd462ec), the orchestrator called setup_skills and setup_mcp_server once each with no prompt argument, and both came back as schema validation errors. It then retried with the argument. Two wasted steps on an otherwise successful hand-off.

Why it is worth a considered fix

prompt is the required field of the shared envelope ArchetypeDelegationTool::parameters_schema emits, so this is not specific to those two tools — it is shared by all 19 delegates. Whatever is done here lands on every hand-off in the product.

orchestrator/agent.toml already records that the envelope's shared argument documentation was deliberately charged once in prompt.md rather than repeated per delegate, to save ~6.8k tokens per turn. Any fix that re-describes the field on every delegate schema gives that back, so the trade-off needs to be made deliberately.

Options, none obviously right

• Default prompt from the user's turn text when omitted — removes the error, but silently invents an argument the model did not supply.
• Name the field in each delegate's description — costs tokens per turn, the thing the current design avoids.
• Leave the schema and improve the recovery so the retry is not a wasted round trip.

Acceptance

Filed for a decision, not a patch. This is a routing-quality change: it deserves an eval comparing hand-off success rates rather than a drive-by, which is why it is last in priority despite being the easiest to "fix".

Found during the #6302 live acceptance run.