feat(config,cli): omnigraph use active-context graph selection

Add omnigraph use <graph>: validate the graph resolves in the merged config (loud
fail otherwise), then write ~/.omnigraph/state/active.yaml ({graph}). The layered
loader reads it as the State layer between Global and Project, so a bare command
targets the active graph — Project still overrides State, State overrides Global.
The State layer is synthetic (sets defaults.graph only) and raises no version/legacy
warnings.

crates/omnigraph-cli/src/main.rs

@@ -26,8 +26,8 @@ use omnigraph_compiler::{
     json_params_to_param_map, lint_query_file,
 };
 use omnigraph_config::{
-    AliasCommand, GraphLocator, OmnigraphConfig, Provenance, ReadOutputFormat,
-    graph_resource_id_for_selection, load_layered_config,
+    ActiveContext, AliasCommand, GraphLocator, OmnigraphConfig, Provenance, ReadOutputFormat,
+    graph_resource_id_for_selection, load_layered_config, write_active_context,
 };
 use omnigraph_policy::{
     PolicyAction, PolicyDecision, PolicyEngine, PolicyRequest, PolicyTestConfig,
@@ -318,6 +318,14 @@ enum Command {
         #[command(subcommand)]
         command: ConfigCommand,
     },
+    /// Set the active graph (writes `~/.omnigraph/state/active.yaml`).
+    Use {
+        /// Graph to make active (must resolve in the merged config).
+        graph: String,
+        /// Project config file (defaults to ./omnigraph.yaml).
+        #[arg(long)]
+        config: Option<PathBuf>,
+    },
 }
 
 /// Operations on the graph registry of a multi-graph server (MR-668).
@@ -3200,6 +3208,17 @@ async fn main() -> Result<()> {
                 )?;
             }
         },
+        Command::Use { graph, config } => {
+            // Validate the graph resolves against the current merged config before
+            // making it the active default (loud fail on an unknown graph).
+            let resolved = load_cli_config(config.as_ref())?;
+            resolved.resolve_graph(None, Some(&graph))?;
+            write_active_context(&ActiveContext {
+                graph: graph.clone(),
+                server: None,
+            })?;
+            println!("active graph set to '{graph}'");
+        }
         Command::Optimize {
             uri,
             target,

crates/omnigraph-cli/tests/cli.rs

@@ -367,6 +367,52 @@ fn config_view_resolved_prints_embedded_and_remote_locators() {
     assert_eq!(remote["graph_id"], "prod");
 }
 
+#[test]
+fn use_sets_active_graph_targeted_by_bare_commands() {
+    let graph_dir = tempdir().unwrap();
+    let graph = graph_path(graph_dir.path());
+    init_graph(&graph);
+
+    // A custom global home that both defines the graph and receives the state file.
+    let home_dir = tempdir().unwrap();
+    write_config(
+        &home_dir.path().join("config.yaml"),
+        &format!(
+            "version: 1\ngraphs:\n  g:\n    storage: {}\n",
+            yaml_string(&graph.to_string_lossy())
+        ),
+    );
+    let empty_cwd = tempdir().unwrap();
+
+    // `use` of an unknown graph fails loudly.
+    output_failure(
+        cli()
+            .env("OMNIGRAPH_HOME", home_dir.path())
+            .current_dir(empty_cwd.path())
+            .arg("use")
+            .arg("nonexistent"),
+    );
+
+    // `use g` records the active context.
+    output_success(
+        cli()
+            .env("OMNIGRAPH_HOME", home_dir.path())
+            .current_dir(empty_cwd.path())
+            .arg("use")
+            .arg("g"),
+    );
+
+    // A bare command (no --graph) now targets the active graph.
+    let output = output_success(
+        cli()
+            .env("OMNIGRAPH_HOME", home_dir.path())
+            .current_dir(empty_cwd.path())
+            .arg("snapshot")
+            .arg("--json"),
+    );
+    assert!(parse_stdout_json(&output).is_object());
+}
+
 #[test]
 fn schema_plan_json_reports_supported_additive_change() {
     let temp = tempdir().unwrap();