feat(cluster): embed schema migration previews in cluster plan

RFC-004 §D7's data-aware preview: for every schema update, plan opens the
live graph read-only and embeds the engine's migration plan (supported flag
+ typed steps) in the change record; the human renderer prints the steps.
Preview failures (unreachable graph, planner error) degrade to the digest
diff with a schema_preview_unavailable warning — planning never blocks.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/omnigraph-cli/src/main.rs

@@ -804,6 +804,17 @@ fn print_cluster_plan_human(output: &PlanOutput) {
         );
         for change in &output.changes {
             println!("  {:?} {}", change.operation, change.resource);
+            if let Some(migration) = &change.migration {
+                if !migration.supported {
+                    println!("      migration UNSUPPORTED:");
+                }
+                for step in &migration.steps {
+                    println!(
+                        "      {}",
+                        serde_json::to_string(step).unwrap_or_else(|_| format!("{step:?}"))
+                    );
+                }
+            }
         }
         if output.changes.is_empty() {
             println!("  no changes");