Export every recorded input, not only the ones that vary

`export runs` dropped a `param.*` column whose value was constant across the
exported runs, so a downstream filter broke depending on which runs the
selection happened to hold. Every input the selection recorded is a column
now; `--params` still narrows it to a sweep's axis. The metrics default is
unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/fluksio/sdk/cli.py

@@ -1394,7 +1394,7 @@ def add_parsers(subparsers: Any) -> None:
         metavar="A,B",
         help=(
             "the inputs to put in columns, dotted into a record "
-            "(default: the ones that vary)"
+            "(default: every input the runs recorded)"
         ),
     )
     sub.add_argument(