refactor: migrate agent configurations and skills to the .agent directory and add project documentation

.agent/skills/nexus-ui-engine/SKILL.md

@@ -0,0 +1,41 @@
+---
+name: nexus-ui-engine
+description: Design System & Component rules for Blazor
+---
+# UI Standards
+
+- **Atomic Design:** 
+  - Build reusable components in `NexusReader.UI.Shared/Components`.
+  - `Atoms`: Base elements (`NexusButton`, `NexusIcon`, `NexusTypography`).
+  - `Molecules`: Small groups (`AiAssistantBubble`, `KnowledgeCheck`).
+  - `Organisms`: High-level sections (`KnowledgeGraph`, `ReaderCanvas`).
+
+- **Styling & Isolation:** 
+  - Mandatory use of scoped CSS (`.razor.css`).
+  - No global CSS except for design tokens in `app.css`.
+  - Use `::deep` only when absolutely necessary to style child components.
+
+- **Design System (Nexus Neon):**
+  - **Color Palette:**
+    - Primary Accent: `--nexus-neon` (`#00ff99`) - Used for borders, highlights, and icons.
+    - Dark Mode: `--nexus-bg` (`#0a0a0a`), `--nexus-card` (`#141414`).
+    - Light Mode: `--nexus-bg` (`#f8f9fa`), `--nexus-card` (`#ffffff`).
+  - **Typography:**
+    - UI Elements: `Inter` (Sans-Serif) for controls, menus, and labels.
+    - Reading Content: `Merriweather` (Serif) for books and articles to ensure high readability.
+  - **Effects:**
+    - Subtle neon glows (`box-shadow: 0 0 15px rgba(0, 255, 153, 0.3)`).
+    - Glassmorphism for overlays and modals.
+
+- **Adaptive Layouts:**
+  - Support `.platform-mobile` and `.platform-desktop` context classes.
+  - Handle safe-area insets (`--safe-area-inset-*`) for mobile devices.
+
+- **Accessibility (A11y):**
+  - Touch Targets: Min `44x44px` on mobile (enforced via CSS variables).
+  - Contrast: Minimum ratio of 4.5:1.
+  - Semantic HTML: Use appropriate tags (`<button>`, `<article>`, `<nav>`).
+
+- **Interactive Flow:**
+  - AI Assistant interactions must be non-blocking and smoothly transition using CSS animations.
+  - Interactive elements must have clear `:hover`, `:active`, and `:focus` states.