feat(ui): add shadcn-vue Select components (#8205)

## Summary

Adds shadcn-vue Select components built on Reka UI primitives with
design system styling.

## Changes

**New Components** (`src/components/ui/select/`):
- `Select.vue` - Root wrapper using `SelectRoot` from Reka UI
- `SelectTrigger.vue` - Styled trigger with chevron icon
- `SelectContent.vue` - Dropdown content with scroll buttons, z-index
3000 for PrimeVue dialog compatibility
- `SelectItem.vue` - Individual option with check icon
- `SelectGroup.vue`, `SelectLabel.vue`, `SelectSeparator.vue` - Grouping
support
- `SelectScrollUpButton.vue`, `SelectScrollDownButton.vue` - Overflow
navigation
- `SelectValue.vue` - Placeholder/value display

**Styling**:
- Uses design tokens (`bg-secondary-background`,
`text-muted-foreground`, `border-border-default`)
- Iconify icons via `icon-[lucide--*]` classes
- Smooth transitions and focus states

**Documentation**:
- Comprehensive Storybook stories covering all variants
- `AGENTS.md` with component creation guidelines

## Testing

- [x] Storybook stories work correctly
- [x] Components build without errors

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8205-feat-ui-add-shadcn-vue-Select-components-2ef6d73d365081fd994ddb1123c063e7)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

src/components/ui/select/SelectTrigger.vue

@@ -0,0 +1,36 @@
+<script setup lang="ts">
+import type { SelectTriggerProps } from 'reka-ui'
+import { SelectIcon, SelectTrigger } from 'reka-ui'
+import type { HTMLAttributes } from 'vue'
+
+import { cn } from '@/utils/tailwindUtil'
+
+const { class: className, ...restProps } = defineProps<
+  SelectTriggerProps & { class?: HTMLAttributes['class'] }
+>()
+</script>
+
+<template>
+  <SelectTrigger
+    v-bind="restProps"
+    :class="
+      cn(
+        'flex h-10 w-full cursor-pointer select-none items-center justify-between',
+        'rounded-lg px-4 py-2 text-sm',
+        'bg-secondary-background text-base-foreground',
+        'border-[2.5px] border-solid border-transparent',
+        'transition-all duration-200 ease-in-out',
+        'focus:border-node-component-border focus:outline-none',
+        'data-[placeholder]:text-muted-foreground',
+        'disabled:cursor-not-allowed disabled:opacity-60',
+        '[&>span]:truncate',
+        className
+      )
+    "
+  >
+    <slot />
+    <SelectIcon as-child>
+      <i class="icon-[lucide--chevron-down] shrink-0 text-muted-foreground" />
+    </SelectIcon>
+  </SelectTrigger>
+</template>