fix(cli): write machine-readable formats directly to stdout bypassing Rich line-wrapping

The format_output() function returned a string that callers passed to
Rich console.print(), which wraps long lines at terminal width.  This
injected literal newline characters into JSON string values (e.g. in
definition_of_done fields), producing invalid JSON that downstream
parsers could not decode (JSONDecodeError: Invalid control character).

For machine-readable formats (json, yaml, plain), format_output() now
writes the rendered output directly to sys.stdout and returns an empty
string.  This preserves the exact serialization from json.dumps/
yaml.dump without Rich text processing artifacts.

Refs: #746

features/steps/plan_diff_artifacts_steps.py

@@ -210,17 +210,23 @@ def step_request_diff_rich(context: Context) -> None:
 
 @when("I request the diff in plain format")
 def step_request_diff_plain(context: Context) -> None:
-    context.diff_output = context.apply_service.diff(context.plan_id, fmt="plain")
+    context.diff_output = _capture_service_output(
+        context.apply_service.diff, context.plan_id, fmt="plain"
+    )
 
 
 @when("I request the diff in JSON format")
 def step_request_diff_json(context: Context) -> None:
-    context.diff_output = context.apply_service.diff(context.plan_id, fmt="json")
+    context.diff_output = _capture_service_output(
+        context.apply_service.diff, context.plan_id, fmt="json"
+    )
 
 
 @when("I request the diff in YAML format")
 def step_request_diff_yaml(context: Context) -> None:
-    context.diff_output = context.apply_service.diff(context.plan_id, fmt="yaml")
+    context.diff_output = _capture_service_output(
+        context.apply_service.diff, context.plan_id, fmt="yaml"
+    )
 
 
 @when("I try to get the diff")
@@ -234,27 +240,40 @@ def step_try_get_diff(context: Context) -> None:
 
 @when("I request the artifacts")
 def step_request_artifacts(context: Context) -> None:
-    context.artifacts_output = context.apply_service.artifacts(context.plan_id)
+    context.artifacts_output = _capture_service_output(
+        context.apply_service.artifacts, context.plan_id
+    )
+
+
+def _capture_service_output(func, *args, **kwargs):
+    """Call a service method capturing stdout (format_output writes there)."""
+    from contextlib import redirect_stdout
+    from io import StringIO
+
+    buf = StringIO()
+    with redirect_stdout(buf):
+        result = func(*args, **kwargs)
+    return result or buf.getvalue().rstrip("\n")
 
 
 @when("I request the artifacts in JSON format")
 def step_request_artifacts_json(context: Context) -> None:
-    context.artifacts_output = context.apply_service.artifacts(
-        context.plan_id, fmt="json"
+    context.artifacts_output = _capture_service_output(
+        context.apply_service.artifacts, context.plan_id, fmt="json"
     )
 
 
 @when("I request the artifacts in plain format")
 def step_request_artifacts_plain(context: Context) -> None:
-    context.artifacts_output = context.apply_service.artifacts(
-        context.plan_id, fmt="plain"
+    context.artifacts_output = _capture_service_output(
+        context.apply_service.artifacts, context.plan_id, fmt="plain"
     )
 
 
 @when("I request the artifacts in YAML format")
 def step_request_artifacts_yaml(context: Context) -> None:
-    context.artifacts_output = context.apply_service.artifacts(
-        context.plan_id, fmt="yaml"
+    context.artifacts_output = _capture_service_output(
+        context.apply_service.artifacts, context.plan_id, fmt="yaml"
     )