feat: simplify Marathon help and enable by default

runtime/codex-rs/app-server/src/marathon_service.rs

@@ -524,9 +524,9 @@ mod tests {
     fn enabled_state_is_persisted_without_auth_material() {
         let directory = tempdir().expect("tempdir");
         let registry = FileAccountRegistry::new(directory.path().join("accounts.json"));
-        assert!(!registry.enabled().expect("default state"));
-        registry.set_enabled(true).expect("enable");
-        assert!(registry.enabled().expect("persisted state"));
+        assert!(registry.enabled().expect("default state"));
+        registry.set_enabled(false).expect("disable");
+        assert!(!registry.enabled().expect("persisted state"));
         let contents = std::fs::read_to_string(registry.path()).expect("registry");
         assert!(!contents.contains("access_token"));
         assert!(!contents.contains("refresh_token"));

runtime/codex-rs/codexmarathon-runtime/src/accounts.rs

@@ -226,7 +226,7 @@ fn validate_metadata_entry(key: &str, value: &str) -> DomainResult<()> {
 }
 
 /// Serializable registry state.
-#[derive(Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
+#[derive(Clone, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub struct RegistryState {
     /// Persisted schema version.
     #[serde(default = "default_registry_version")]
@@ -235,9 +235,9 @@ pub struct RegistryState {
     #[serde(default, skip_serializing_if = "String::is_empty")]
     pub active_account_id: String,
     /// Whether the native Marathon service may perform account transitions.
-    /// Defaults to disabled for existing registries so adding the service is
-    /// behavior preserving until an operator explicitly enables it.
-    #[serde(default)]
+    /// New registries enable account management; an explicitly persisted value
+    /// continues to win when an existing registry is loaded.
+    #[serde(default = "default_registry_enabled")]
     pub enabled: bool,
     /// Records keyed by stable account ID.
     #[serde(default)]
@@ -248,6 +248,21 @@ fn default_registry_version() -> u32 {
     REGISTRY_VERSION
 }
 
+fn default_registry_enabled() -> bool {
+    true
+}
+
+impl Default for RegistryState {
+    fn default() -> Self {
+        Self {
+            version: REGISTRY_VERSION,
+            active_account_id: String::new(),
+            enabled: default_registry_enabled(),
+            accounts: BTreeMap::new(),
+        }
+    }
+}
+
 /// Storage-neutral account registry surface.
 pub trait AccountStore: Send + Sync {
     /// Insert a new account and reject duplicate IDs.

runtime/codex-rs/tui/src/chatwidget/slash_dispatch.rs

@@ -1228,7 +1228,7 @@ impl ChatWidget {
         };
         self.add_info_message(
             format!(
-                "Native Marathon is built into Codex.\n{current}\n\nCommands:\n  /marathon on | off\n  /marathon status\n  /marathon auto-reset on | off | status\n  /marathon login <alias>   (choose browser link or device code)\n  /marathon import <alias>\n  /marathon switch <alias>\n\nAutomatic reset uses a provider-supported reset action only after every eligible managed account reports zero weekly quota. It never guesses a reset time. For a headless server, choose device code and finish sign-in from another machine. No separate codexmarathon command is required."
+                "Native Marathon is built into Codex.\n{current}\n\nCommands:\n  /marathon on (enable account switching)\n  /marathon off (disable account switching)\n  /marathon status (show accounts and active account)\n  /marathon auto-reset on | off | status (configure automatic quota reset)\n  /marathon login <alias> (choose browser link or device code)\n  /marathon import <alias> (save the current account under an alias)\n  /marathon switch <alias> (switch to a saved account)\n\nStart by importing your current account with /marathon import <alias>. For a headless server, choose device code and finish sign-in from another machine. No separate codexmarathon command is required."
             ),
             Some("Use ↑ to recall this help".to_string()),
         );