cluster: deprecate DefaultSubmitArgs (no longer prepended)

cluster.DefaultSubmitArgs is now deprecated. It used to be prepended to
spark_submit Tool calls, but since 7920dc9 the spark-submit builder
constructs argv from the structured fields in the tool call. The field is
still accepted by the admin API and still persisted to the DB, so this is
a non-breaking change.

This commit deliberately does NOT remove:
- the Cluster.DefaultSubmitArgs field in internal/cluster/types.go
- the default_submit_args DB column
- the admin API read/write support in internal/admin/router.go
- existing storage/repo tests

Runtime now logs a one-time slog.Info when the PUT handler receives a
non-empty default_submit_args value, warning operators that the field is
ignored by spark_submit and that they should use spark_conf / extra_args
instead.

Follow-up removal should touch:
- the Cluster struct field and its JSON tag
- storage/cluster_repo.go scan/insert/update SQL and params
- admin/router.go JSON round-tripping
- storage/cluster_repo_test.go assertions
- the DB migration dropping the default_submit_args column

Co-Authored-By: Claude <noreply@anthropic.com>

internal/admin/router.go

@@ -5,6 +5,7 @@ package admin
 import (
 	"encoding/json"
 	"errors"
+	"log/slog"
 	"net/http"
 	"strconv"
 
@@ -170,6 +171,10 @@ func updateCluster(repo *storage.ClusterRepo, auditRepo *audit.Repo) gin.Handler
 			merged.DefaultSubmitArgs = decodeStringSlice(v)
 		}
 
+		if len(merged.DefaultSubmitArgs) > 0 {
+			slog.Default().Info("admin: cluster sets default_submit_args which is deprecated; the spark_submit Tool no longer prepends these args. Configure flags via spark_conf / extra_args in the structured tool call.", "cluster_id", merged.ID)
+		}
+
 		// Re-validate after merging so a missing auth_type on a fresh
 		// cluster (defaulted by DB to "none") doesn't get clobbered.
 		if err := validateClusterUpdate(&merged); err != nil {