fix(tools): gate generate_image behind setting and tool whitelist

generate_image was registered as a custom tool and force-activated via the alwaysInclude list in createAgentSession, so it survived --no-tools (empty toolNames) and any explicit whitelist that omitted it. There was also no generate_image.enabled setting, so /settings had no toggle.

Add a generate_image.enabled setting and only register the tool when enabled and either no whitelist is given or it names generate_image.

Fixes #5305

packages/coding-agent/src/config/settings-schema.ts

@@ -3528,6 +3528,16 @@ export const SETTINGS_SCHEMA = {
 			description: "Enable the tts tool for on-device (Kokoro) or xAI Grok Voice speech-file synthesis",
 		},
 	},
+	"generate_image.enabled": {
+		type: "boolean",
+		default: true,
+		ui: {
+			tab: "tools",
+			group: "Available Tools",
+			label: "Generate Image",
+			description: "Enable the generate_image tool for text-to-image generation and editing",
+		},
+	},
 
 	"inspect_image.enabled": {
 		type: "boolean",