exp run: git add param files that will be modified.

Only if they are currently untracked and `--temp` or `--queue` is passed.

Closes #7842

dvc/repo/experiments/run.py

@@ -11,7 +11,7 @@
 
 
 @locked
-def run(  # noqa: C901
+def run(  # noqa: C901, PLR0912
     repo,
     targets: Optional[Iterable[str]] = None,
     params: Optional[Iterable[str]] = None,
@@ -36,6 +36,18 @@ def run(  # noqa: C901
         from dvc.utils.hydra import to_hydra_overrides
 
         path_overrides = to_path_overrides(params)
+
+        if tmp_dir or queue:
+            untracked = repo.scm.untracked_files()
+            for path in path_overrides:
+                if path in untracked:
+                    logger.debug(
+                        "'%s' is currently untracked but will be modified by DVC. "
+                        "Adding it to git.",
+                        path,
+                    )
+                    repo.scm.add([path])
+
         hydra_sweep = any(
             x.is_sweep_override()
             for param_file in path_overrides

tests/func/experiments/test_queue.py

@@ -92,3 +92,19 @@ def test_queue_remove_done(
     assert set(celery_queue.clear(success=True)) == set(success_tasks[:2])
 
     assert celery_queue.status() == []
+
+
+def test_queue_doesnt_remove_untracked_params_file(tmp_dir, dvc, scm):
+    """Regression test for https://github.com/iterative/dvc/issues/7842"""
+    tmp_dir.gen("params.yaml", "foo: 1")
+    stage = dvc.run(cmd="echo ${foo}", params=["foo"], name="echo-foo")
+    scm.add(
+        [
+            "dvc.yaml",
+            "dvc.lock",
+            ".gitignore",
+        ]
+    )
+    scm.commit("init")
+    dvc.experiments.run(stage.addressing, params=["foo=2"], queue=True)
+    assert (tmp_dir / "params.yaml").exists()