fix(config): error when OMNIGRAPH_CONFIG names a missing file

An explicitly-set OMNIGRAPH_CONFIG pointing at a missing path was silently dropped
(no global layer, confusing downstream 'graph not found'). Bail naming the file, so an
explicitly-named global config is required like --config; the default ~/.omnigraph path
stays optional.

crates/omnigraph-cli/tests/cli.rs

@@ -417,6 +417,26 @@ fn use_sets_active_graph_targeted_by_bare_commands() {
     assert!(parse_stdout_json(&output).is_object());
 }
 
+#[test]
+fn explicit_omnigraph_config_pointing_at_missing_file_errors() {
+    // A typo'd OMNIGRAPH_CONFIG must fail loudly naming the file, not silently
+    // load no global layer.
+    let empty_cwd = tempdir().unwrap();
+    let missing = empty_cwd.path().join("does-not-exist.yaml");
+    let output = output_failure(
+        cli()
+            .current_dir(empty_cwd.path())
+            .env("OMNIGRAPH_CONFIG", &missing)
+            .arg("config")
+            .arg("view"),
+    );
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("OMNIGRAPH_CONFIG") && stderr.contains("does-not-exist.yaml"),
+        "missing OMNIGRAPH_CONFIG must error naming the file: {stderr}"
+    );
+}
+
 #[test]
 fn schema_plan_json_reports_supported_additive_change() {
     let temp = tempdir().unwrap();

crates/omnigraph-config/src/lib.rs

@@ -1158,6 +1158,19 @@ pub struct LayeredConfig {
 pub fn load_layered_config(project_config_path: Option<&PathBuf>) -> Result<LayeredConfig> {
     let cwd = env::current_dir()?;
     let global = global_config_file();
+    // An explicitly-named global file (via `OMNIGRAPH_CONFIG`) is *required*: a
+    // typo must error loudly, not silently load no global layer. A missing
+    // default-location file (`~/.omnigraph/config.yaml`) stays optional.
+    if env::var_os("OMNIGRAPH_CONFIG").is_some() {
+        if let Some(path) = &global {
+            if !path.exists() {
+                bail!(
+                    "OMNIGRAPH_CONFIG points at a missing file: {}",
+                    path.display()
+                );
+            }
+        }
+    }
     let active = active_context_file();
     load_layered_config_in(
         &cwd,