refactor: eliminate code duplication with shared utilities

Created 5 new utility modules in apps/server/src/lib/ to eliminate ~320 lines of duplicated code:
- image-handler.ts: Centralized image processing (MIME types, base64, content blocks)
- prompt-builder.ts: Standardized prompt building with image attachments
- model-resolver.ts: Model alias resolution and provider routing
- conversation-utils.ts: Conversation history processing for providers
- error-handler.ts: Error classification and user-friendly messages

Updated services and providers to use shared utilities:
- agent-service.ts: -51 lines (removed duplicate image handling, model logic)
- auto-mode-service.ts: -75 lines (removed MODEL_MAP, duplicate utilities)
- claude-provider.ts: -10 lines (uses conversation-utils)
- codex-provider.ts: -5 lines (uses conversation-utils)

Added comprehensive documentation:
- docs/server/utilities.md: Complete reference for all 9 lib utilities
- docs/server/providers.md: Provider architecture guide with examples

Benefits:
- Single source of truth for critical business logic
- Improved maintainability and testability
- Consistent behavior across services and providers
- Better documentation for future development

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/server/src/lib/image-handler.ts

@@ -0,0 +1,135 @@
+/**
+ * Image handling utilities for processing image files
+ *
+ * Provides utilities for:
+ * - MIME type detection based on file extensions
+ * - Base64 encoding of image files
+ * - Content block generation for Claude SDK format
+ * - Path resolution (relative/absolute)
+ */
+
+import fs from "fs/promises";
+import path from "path";
+
+/**
+ * MIME type mapping for image file extensions
+ */
+const IMAGE_MIME_TYPES: Record<string, string> = {
+  ".jpg": "image/jpeg",
+  ".jpeg": "image/jpeg",
+  ".png": "image/png",
+  ".gif": "image/gif",
+  ".webp": "image/webp",
+} as const;
+
+/**
+ * Image data with base64 encoding and metadata
+ */
+export interface ImageData {
+  base64: string;
+  mimeType: string;
+  filename: string;
+  originalPath: string;
+}
+
+/**
+ * Content block for image (Claude SDK format)
+ */
+export interface ImageContentBlock {
+  type: "image";
+  source: {
+    type: "base64";
+    media_type: string;
+    data: string;
+  };
+}
+
+/**
+ * Get MIME type for an image file based on extension
+ *
+ * @param imagePath - Path to the image file
+ * @returns MIME type string (defaults to "image/png" for unknown extensions)
+ */
+export function getMimeTypeForImage(imagePath: string): string {
+  const ext = path.extname(imagePath).toLowerCase();
+  return IMAGE_MIME_TYPES[ext] || "image/png";
+}
+
+/**
+ * Read an image file and convert to base64 with metadata
+ *
+ * @param imagePath - Path to the image file
+ * @returns Promise resolving to image data with base64 encoding
+ * @throws Error if file cannot be read
+ */
+export async function readImageAsBase64(imagePath: string): Promise<ImageData> {
+  const imageBuffer = await fs.readFile(imagePath);
+  const base64Data = imageBuffer.toString("base64");
+  const mimeType = getMimeTypeForImage(imagePath);
+
+  return {
+    base64: base64Data,
+    mimeType,
+    filename: path.basename(imagePath),
+    originalPath: imagePath,
+  };
+}
+
+/**
+ * Convert image paths to content blocks (Claude SDK format)
+ * Handles both relative and absolute paths
+ *
+ * @param imagePaths - Array of image file paths
+ * @param workDir - Optional working directory for resolving relative paths
+ * @returns Promise resolving to array of image content blocks
+ */
+export async function convertImagesToContentBlocks(
+  imagePaths: string[],
+  workDir?: string
+): Promise<ImageContentBlock[]> {
+  const blocks: ImageContentBlock[] = [];
+
+  for (const imagePath of imagePaths) {
+    try {
+      // Resolve to absolute path if needed
+      const absolutePath = workDir && !path.isAbsolute(imagePath)
+        ? path.join(workDir, imagePath)
+        : imagePath;
+
+      const imageData = await readImageAsBase64(absolutePath);
+
+      blocks.push({
+        type: "image",
+        source: {
+          type: "base64",
+          media_type: imageData.mimeType,
+          data: imageData.base64,
+        },
+      });
+    } catch (error) {
+      console.error(`[ImageHandler] Failed to load image ${imagePath}:`, error);
+      // Continue processing other images
+    }
+  }
+
+  return blocks;
+}
+
+/**
+ * Build a list of image paths for text prompts
+ * Formats image paths as a bulleted list for inclusion in text prompts
+ *
+ * @param imagePaths - Array of image file paths
+ * @returns Formatted string with image paths, or empty string if no images
+ */
+export function formatImagePathsForPrompt(imagePaths: string[]): string {
+  if (imagePaths.length === 0) {
+    return "";
+  }
+
+  let text = "\n\nAttached images:\n";
+  for (const imagePath of imagePaths) {
+    text += `- ${imagePath}\n`;
+  }
+  return text;
+}