Let a jsonl export keep a record a value

`--format jsonl` existed, but `_cell` ran json.dumps at row-build time,
before a format was chosen — so a nested value was a string by then and
jsonl only re-escaped it, leaving a consumer against csv's 128KB field
limit either way. Stringifying moved to the csv writer, so csv is
byte-identical and jsonl carries json. The runs TUI followed, or a record
would draw as a Python repr.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KYM38KSb4V4v2T71eifnZv

backend/fluksio/sdk/cli.py

@@ -1128,9 +1128,20 @@ def _write_rows(rows: list[dict[str, Any]], fmt: str, out: str, hint: str = "")
         if fmt == "jsonl":
             handle.writelines(json.dumps(row) + "\n" for row in rows)
         else:
+            # The wire is jsonl either way (see `Client._export`), so a record
+            # a jsonl row keeps as a value is still one here — stringify it
+            # for the csv cell it has to sit in.
             writer = csv.DictWriter(handle, fieldnames=list(rows[0]))
             writer.writeheader()
-            writer.writerows(rows)
+            writer.writerows(
+                {
+                    key: value
+                    if value is None or isinstance(value, (str, int, float, bool))
+                    else json.dumps(value)
+                    for key, value in row.items()
+                }
+                for row in rows
+            )
     return 0