Surface document and artifact variables in the prompt variable pickers

Add the run input-documents variable to the workflow builder's variable
dropdown (workflow input) and the artifact-by-id lookup to both the workflow
builder and the agent prompt builder, so the variables exposed by the shared
template namespaces are discoverable. agent.input_documents is workflow-only
(injected from run state) and stays out of the agent builder, which would
render it empty.

frontend/src/agents/workflow/components/PromptTextArea.tsx

@@ -62,6 +62,11 @@ const GLOBAL_CONTEXT_VARIABLES: WorkflowVariable[] = [
     templatePath: 'system.user_id',
     section: 'Global context',
   },
+  {
+    label: 'artifacts.artifact(id)',
+    templatePath: 'artifacts.artifact(id)',
+    section: 'Global context',
+  },
 ];
 
 function toAgentTemplatePath(variableName: string): string {
@@ -109,6 +114,11 @@ function extractUpstreamVariables(
       templatePath: 'agent.chat_history',
       section: 'Workflow input',
     },
+    {
+      label: 'agent.input_documents',
+      templatePath: 'agent.input_documents',
+      section: 'Workflow input',
+    },
     ...GLOBAL_CONTEXT_VARIABLES,
   ];
   const seen = new Set(variables.map((variable) => variable.templatePath));

frontend/src/locale/en.json

@@ -932,7 +932,8 @@
         "systemTime": "Current time (HH:MM:SS)",
         "systemTimestamp": "ISO 8601 timestamp",
         "systemRequestId": "Unique request identifier",
-        "systemUserId": "Current user ID"
+        "systemUserId": "Current user ID",
+        "artifactsLookup": "Artifact metadata by id"
       },
       "learnAboutPrompts": "Learn about Prompts →",
       "publicPromptEditDisabled": "Public prompts cannot be edited",

frontend/src/preferences/PromptsModal.tsx

@@ -89,6 +89,10 @@ const systemVariableOptionDefinitions = [
     labelKey: 'modals.prompts.systemVariableOptions.systemUserId',
     value: 'system.user_id',
   },
+  {
+    labelKey: 'modals.prompts.systemVariableOptions.artifactsLookup',
+    value: 'artifacts.artifact(id)',
+  },
 ];
 
 const buildSystemVariableOptions = (translate: (key: string) => string) =>