feat(workflow): surface manual-trigger payload + metadata in variable picker (#21692)

## Summary

Step 2 of the manual-trigger output schema restructuring (expand →
display → migrate → contract).

Builds on the now-merged #21676 (which expanded the runtime payload to
serve `payload` and `metadata` siblings at the trigger root). This PR
**surfaces** those in the variable picker as nested, expandable nodes:

- `trigger.payload.{record fields}` — the record(s) that triggered the
run
- `trigger.metadata.workspaceMemberId` — who triggered it

The flat root fields (`trigger.id`, etc.) remain available, so existing
saved variable references keep working until a later migration phase
moves them.

### Changes
- **twenty-shared**: metadata/payload label constants +
`build-manual-trigger-metadata-node` util + barrel exports.
- **twenty-front**: `computeStepOutputSchema` MANUAL branch now nests
`payload` (RecordNode for SINGLE_RECORD, array Node for BULK_RECORDS,
omitted for GLOBAL) and `metadata`; `ManualTriggerOutputSchema` type
updated to `{ payload?; metadata }`.
- **twenty-server**: `computeTriggerOutputSchemaFromAvailability`
mirrors the same nested shape for server-side validation.

The key is `metadata` (not `_metadata`) — custom fields can't start with
`_`, so collision risk was deemed acceptable.

## Test plan
- [x] `npx nx build twenty-shared`
- [x] `computeStepOutputSchema` unit tests pass (55)
- [x] Manual: create a manual-trigger workflow (GLOBAL / single-record /
bulk), confirm the picker shows `payload` and `metadata` as expandable
folders and that selecting a field yields `{{trigger.payload.<field>}}`
/ `{{trigger.metadata.workspaceMemberId}}`

> Note: server typecheck has pre-existing unrelated failures on main
(Stripe billing mocks, gmail mocks); none touch workflow files.

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packages/twenty-server/src/modules/workflow/workflow-builder/workflow-schema/workflow-schema.workspace-service.ts

@@ -4,12 +4,16 @@ import { isString } from '@sniptt/guards';
 import { isDefined, isValidVariable } from 'twenty-shared/utils';
 import {
   BaseOutputSchemaV2,
+  buildManualTriggerMetadataNode,
   BulkRecordsAvailability,
   extractRawVariableNamePart,
   GlobalAvailability,
   navigateOutputSchemaProperty,
   SingleRecordAvailability,
   TRIGGER_STEP_ID,
+  WORKFLOW_TRIGGER_METADATA_KEY,
+  WORKFLOW_TRIGGER_PAYLOAD_KEY,
+  WORKFLOW_TRIGGER_PAYLOAD_LABEL,
   WorkflowActionType,
 } from 'twenty-shared/workflow';
 
@@ -291,14 +295,28 @@ export class WorkflowSchemaWorkspaceService {
     workspaceId: string;
   }): Promise<OutputSchema> {
     if (availability.type === 'GLOBAL') {
-      return {};
+      return {
+        [WORKFLOW_TRIGGER_METADATA_KEY]: buildManualTriggerMetadataNode(),
+      };
     }
 
     if (availability.type === 'SINGLE_RECORD') {
-      return this.computeRecordOutputSchema({
+      const recordOutputSchema = await this.computeRecordOutputSchema({
         objectType: availability.objectNameSingular,
         workspaceId,
       });
+
+      const payload: Node = {
+        isLeaf: false,
+        type: 'object',
+        label: WORKFLOW_TRIGGER_PAYLOAD_LABEL,
+        value: recordOutputSchema,
+      };
+
+      return {
+        [WORKFLOW_TRIGGER_PAYLOAD_KEY]: payload,
+        [WORKFLOW_TRIGGER_METADATA_KEY]: buildManualTriggerMetadataNode(),
+      };
     }
 
     if (availability.type === 'BULK_RECORDS') {
@@ -308,15 +326,25 @@ export class WorkflowSchemaWorkspaceService {
           workspaceId,
         );
 
-      return {
-        [objectMetadataInfo.flatObjectMetadata.namePlural]: {
-          label: objectMetadataInfo.flatObjectMetadata.labelPlural,
-          isLeaf: true,
-          type: 'array',
-          value:
-            'Array of ' + objectMetadataInfo.flatObjectMetadata.labelPlural,
+      const payload: Node = {
+        isLeaf: false,
+        type: 'object',
+        label: WORKFLOW_TRIGGER_PAYLOAD_LABEL,
+        value: {
+          [objectMetadataInfo.flatObjectMetadata.namePlural]: {
+            label: objectMetadataInfo.flatObjectMetadata.labelPlural,
+            isLeaf: true,
+            type: 'array',
+            value:
+              'Array of ' + objectMetadataInfo.flatObjectMetadata.labelPlural,
+          },
         },
       };
+
+      return {
+        [WORKFLOW_TRIGGER_PAYLOAD_KEY]: payload,
+        [WORKFLOW_TRIGGER_METADATA_KEY]: buildManualTriggerMetadataNode(),
+      };
     }
 
     return {};