Add Description field to expression entries similar to decision table

[yash9323]

Why

Expression nodes often contain complex business logic (data transformations, aggregations,
conditional calculations). When reviewing or debugging a graph, it's hard to remember what each
expression does without reading the code.

Changes

• Added _description?: string to ExpressionEntry type
• Added Description column in expression list (header + input field per row)
• Updated schema validation for expression node expressions
• Added separator border between expression code and description
• Increased description column width (min 280px, flex 1.2fr)

Tests

• Build passes
• TypeScript typecheck passes
• Lint passes
• Description field renders in Storybook
• Description is editable with edit:full permission
• Description is read-only with view/edit:values permission