fix: don't force-load explicitly disabled builtin extensions

The ACP server unconditionally pushed every builtin (e.g. developer) into
each new session before consulting the extensions config. A user who
disabled developer in Settings still got shell/edit/write/tree/read_image
handed to the model, because the config-derived list only *adds* enabled
extensions and never removes the force-added builtin.

Skip a builtin when the config has an explicit enabled: false entry for it.
Missing entries still load (fresh installs / default-on bundled extensions
behave as before), so this only changes the explicitly-disabled case.

Adds config::extensions::configured_enabled_state /
is_extension_explicitly_disabled and unit tests.

crates/goose/src/acp/server.rs

@@ -16,7 +16,9 @@ use crate::agents::{
     Agent, AgentConfig, ExtensionConfig, ExtensionLoadResult, GoosePlatform, SessionConfig,
 };
 use crate::config::base::CONFIG_YAML_NAME;
-use crate::config::extensions::get_enabled_extensions_with_config;
+use crate::config::extensions::{
+    get_enabled_extensions_with_config, is_extension_explicitly_disabled,
+};
 use crate::config::paths::Paths;
 use crate::config::permission::PermissionManager;
 use crate::config::{Config, GooseMode};
@@ -1028,7 +1030,11 @@ impl GooseAcpAgent {
     ) -> Result<Vec<ExtensionConfig>, agent_client_protocol::Error> {
         let mut extensions = Vec::new();
         for builtin in &self.builtins {
-            push_or_replace_extension(&mut extensions, builtin_to_extension_config(builtin));
+            let builtin_config = builtin_to_extension_config(builtin);
+            if is_extension_explicitly_disabled(config, &builtin_config.name()) {
+                continue;
+            }
+            push_or_replace_extension(&mut extensions, builtin_config);
         }
 
         if let Some(recipe_extensions) = recipe_extensions {

crates/goose/src/config/extensions.rs

@@ -199,6 +199,27 @@ pub fn is_extension_enabled(key: &str) -> bool {
     extensions.get(key).map(|e| e.enabled).unwrap_or(false)
 }
 
+/// Returns the configured enabled state for an extension, or `None` when the
+/// extension has no entry in the config at all.
+///
+/// This lets callers distinguish "not configured" (e.g. a fresh install where a
+/// bundled builtin should still load by default) from "explicitly turned off".
+pub fn configured_enabled_state(config: &Config, name: &str) -> Option<bool> {
+    let extensions = get_extensions_map_with_config(config);
+    let key = name_to_key(name);
+    extensions
+        .values()
+        .find(|entry| entry.config.name() == name)
+        .or_else(|| extensions.get(&key))
+        .map(|entry| entry.enabled)
+}
+
+/// Returns true only when an extension has an explicit config entry that is
+/// disabled. Missing entries return false so default-on builtins keep loading.
+pub fn is_extension_explicitly_disabled(config: &Config, name: &str) -> bool {
+    matches!(configured_enabled_state(config, name), Some(false))
+}
+
 pub fn get_enabled_extensions() -> Vec<ExtensionConfig> {
     get_all_extensions()
         .into_iter()
@@ -664,4 +685,39 @@ extensions:
             other_keys
         );
     }
+
+    #[test]
+    fn test_configured_enabled_state_unknown_extension_is_none() {
+        let (config, _config_file, _secrets_file) = test_config("");
+
+        assert_eq!(
+            configured_enabled_state(&config, "not_a_real_extension"),
+            None
+        );
+        assert!(!is_extension_explicitly_disabled(
+            &config,
+            "not_a_real_extension"
+        ));
+    }
+
+    #[test]
+    fn test_default_enabled_platform_extension_is_not_explicitly_disabled() {
+        let (config, _config_file, _secrets_file) = test_config("");
+
+        assert_eq!(configured_enabled_state(&config, "developer"), Some(true));
+        assert!(!is_extension_explicitly_disabled(&config, "developer"));
+    }
+
+    #[test]
+    fn test_configured_enabled_state_reflects_saved_entry() {
+        let (config, _config_file, _secrets_file) = test_config("");
+        set_extension_with_config(&config, builtin_entry("developer", false));
+
+        assert_eq!(configured_enabled_state(&config, "developer"), Some(false));
+        assert!(is_extension_explicitly_disabled(&config, "developer"));
+
+        set_extension_enabled_with_config(&config, "developer", true);
+        assert_eq!(configured_enabled_state(&config, "developer"), Some(true));
+        assert!(!is_extension_explicitly_disabled(&config, "developer"));
+    }
 }