feat: add @automaker/git-utils package

ELIMINATES routes/common.ts: Extract all git operations (382 lines) into dedicated package.

- Extract git status parsing (parseGitStatus, isGitRepo)
- Extract diff generation (generateSyntheticDiffForNewFile, etc.)
- Extract repository analysis (getGitRepositoryDiffs)
- Handle both git repos and non-git directories
- Support binary file detection
- Generate synthetic diffs for untracked files

Split into logical modules:
- types.ts: Constants and interfaces
- status.ts: Git status operations
- diff.ts: Diff generation utilities

This package will replace:
- apps/server/src/routes/common.ts (to be deleted)

Dependencies: @automaker/types, @automaker/utils

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

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

libs/git-utils/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@automaker/git-utils",
+  "version": "1.0.0",
+  "description": "Git operations utilities for AutoMaker",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "scripts": {
+    "build": "tsc",
+    "watch": "tsc --watch"
+  },
+  "keywords": ["automaker", "git", "utils"],
+  "author": "",
+  "license": "MIT",
+  "dependencies": {
+    "@automaker/types": "^1.0.0",
+    "@automaker/utils": "^1.0.0"
+  },
+  "devDependencies": {
+    "@types/node": "^22.10.5",
+    "typescript": "^5.7.3"
+  }
+}