feat: emit stable json errors

docs/compatibility.md

@@ -36,6 +36,12 @@ explicit migration expectation before scripts depend on them.
 top-level response variant name and existing field names keep their meaning
 within a major release.
 
+Execution failures in `--json` or `--jsonl` mode return a stable JSON error
+object on stdout with `type: "error"`, a machine-readable `code`, a human
+`message`, full `detail`, and an optional `hint` derived from operator
+recovery output. Common codes include `daemon_unavailable`, `unauthorized`,
+`peer_not_found`, `resource_not_found`, `invalid_input`, and `command_failed`.
+
 Backward-compatible JSON changes include:
 
 - Adding nullable or optional fields.