docs(automation): document the flat screen-field shape and visibleWhen

The flows guide covered the `object-form` screen step but never the default
flat `fields` list, so the shape most screens actually use was undocumented —
including `visibleWhen`, which this branch just made live on the wire.

Records the three things an author cannot infer from the type: the predicate is
bare CEL over the screen's own field names (NOT the `{var}` template dialect the
rest of a node's config uses), `required` on a conditional field means "required
when shown", and a `required` boolean needs `defaultValue: false` or the user
cannot express "no" by leaving the box clear. Also notes that a broken predicate
fails open, since nothing validates it at author time yet.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0122xvfanLmniNbsAPtg5hk3

Author: claude

content/docs/automation/flows.mdx

@@ -210,9 +210,50 @@ rather than evaluating an arbitrary JavaScript string:
 }
 ```
 
+**Screen (flat fields):**
+
+The default shape. Each field is collected as a **bare flow variable**, so a
+later node reads `{subject}`, not `{screen_1.subject}`.
+
+```typescript
+{
+  id: 'screen_1',
+  type: 'screen',
+  label: 'Conversion Details',
+  config: {
+    fields: [
+      { name: 'createOpportunity', label: 'Create Opportunity?', type: 'boolean',
+        required: true, defaultValue: false },
+      { name: 'opportunityName', label: 'Opportunity Name', type: 'text',
+        required: true,
+        visibleWhen: 'createOpportunity == true' },
+    ],
+  },
+}
+```
+
+`visibleWhen` is **bare CEL over the screen's own field names** — not the `{var}`
+template dialect the rest of a node's `config` uses. The client re-evaluates it
+as the user types, against the values collected so far, which is why it cannot
+be a server-interpolated template. Omit it for a field that is always visible.
+
+A hidden field is not collected, and is **not enforced as `required`** — so
+`required` on a `visibleWhen` field means "required *when shown*". That is what
+lets the example above ask for an opportunity name only when one is being
+created.
+
+Two things worth knowing:
+
+- **Give a `required` boolean a `defaultValue`.** An untouched checkbox holds no
+  value at all, which counts as unanswered — without `defaultValue: false` the
+  user cannot express "no" by leaving it clear.
+- **A broken predicate fails open** (the field stays visible) rather than hiding
+  an input the flow may be waiting on. Nothing validates the expression at
+  author time yet, so a typo shows up as a field that never hides.
+
 **Screen (object form):**
 
-A `screen` node normally renders a flat `fields` list. Set `config.objectName`
+Set `config.objectName`
 to instead render an object's **entire** create/edit form — including any
 master-detail child grids (`inlineEdit`) — as one wizard step. The client
 renders the object form and, on save, **persists the record itself** (parent +