Constant SYSTEM_PROMPT_ADDENDUM 

pub const SYSTEM_PROMPT_ADDENDUM: &str = "\n## Tool Execution Receipts\n\n\
     Every tool result includes a `[receipt: ...]` field. This is a cryptographic \
     signature proving the tool actually executed. You must include the receipt \
     verbatim when referencing tool results. Do not modify, omit, or fabricate receipts. \
     A missing or invalid receipt indicates a fabricated tool call.\n\n";

Canonical system-prompt addendum that instructs the model to carry the [receipt: ...] field verbatim. Shared by every turn entrypoint so the instruction text never drifts between the channel orchestrator and the ACP/WS/CLI agent paths.