feat: add enabled_macros config field at global, role, agent, and session levels

Mirrors the enabled_skills plumbing per plans/custom-commands-design.md §5:
- global: Config + AppConfig structs, from_config copy, and the
  COYOTE_ENABLED_MACROS env-override arm (csv_to_vec parsing)
- role: frontmatter via parse_string_or_array (list or csv string),
  plus the export() mirror so Role::save round-trips the field
- agent (non-graph): plain serde on AgentConfig; graph.yaml silently
  ignores the key (pinned by test, no field on Graph by design)
- session: plain serde with csv-or-vec deserializer

Empty list/string deserializes to Some([]) (explicit zero), distinct
from absent/null (None) — pinned by tests at every level, including
the env arm (serial-fenced against the from_config tests, which read
the process env via load_envs).

src/config/agent.rs

@@ -744,6 +744,8 @@ pub struct AgentConfig {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub enabled_skills: Option<Vec<String>>,
     #[serde(skip_serializing_if = "Option::is_none")]
+    pub enabled_macros: Option<Vec<String>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     pub continuation_prompt: Option<String>,
     #[serde(default)]
     pub instructions: String,
@@ -1225,6 +1227,30 @@ variables:
         assert!(config.top_p.is_none());
     }
 
+    #[test]
+    fn agent_config_enabled_macros_absent_is_none() {
+        let yaml = "name: minimal\ninstructions: hi\n";
+        let config: AgentConfig = serde_yaml::from_str(yaml).unwrap();
+
+        assert_eq!(config.enabled_macros, None);
+    }
+
+    #[test]
+    fn agent_config_enabled_macros_empty_list_is_some_empty() {
+        let yaml = "name: minimal\ninstructions: hi\nenabled_macros: []\n";
+        let config: AgentConfig = serde_yaml::from_str(yaml).unwrap();
+
+        assert_eq!(config.enabled_macros, Some(vec![]));
+    }
+
+    #[test]
+    fn agent_config_enabled_macros_list() {
+        let yaml = "name: minimal\ninstructions: hi\nenabled_macros:\n  - a\n";
+        let config: AgentConfig = serde_yaml::from_str(yaml).unwrap();
+
+        assert_eq!(config.enabled_macros, Some(vec!["a".to_string()]));
+    }
+
     #[test]
     fn agent_config_with_model() {
         let yaml =