test: Add comprehensive unit tests for shared packages

Add 88 new unit tests covering critical business logic in shared packages:

- libs/git-utils/tests/diff.test.ts (22 tests)
  * Synthetic diff generation for new files
  * Binary file handling
  * Large file handling
  * Untracked file diff appending
  * Directory file listing with exclusions
  * Non-git directory handling

- libs/dependency-resolver/tests/resolver.test.ts (30 tests)
  * Topological sorting with dependencies
  * Priority-aware ordering
  * Circular dependency detection
  * Missing dependency tracking
  * Blocked feature detection
  * Complex dependency graphs

- libs/utils/tests/error-handler.test.ts (36 tests)
  * Abort error detection
  * Cancellation error detection
  * Authentication error detection
  * Error classification logic
  * User-friendly error messages

All tests use vitest and follow best practices with proper setup/teardown.

Resolves PR review issue #1 (HIGH PRIORITY)

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

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

libs/utils/package.json

@@ -6,7 +6,9 @@
   "types": "dist/index.d.ts",
   "scripts": {
     "build": "tsc",
-    "watch": "tsc --watch"
+    "watch": "tsc --watch",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "keywords": ["automaker", "utils"],
   "author": "AutoMaker Team",
@@ -16,6 +18,7 @@
   },
   "devDependencies": {
     "@types/node": "^22.10.5",
-    "typescript": "^5.7.3"
+    "typescript": "^5.7.3",
+    "vitest": "^4.0.16"
   }
 }