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/src/index.ts

@@ -0,0 +1,26 @@
+/**
+ * @automaker/git-utils
+ * Git operations utilities for AutoMaker
+ */
+
+// Export types and constants
+export {
+  BINARY_EXTENSIONS,
+  GIT_STATUS_MAP,
+  type FileStatus,
+} from './types';
+
+// Export status utilities
+export {
+  isGitRepo,
+  parseGitStatus,
+} from './status';
+
+// Export diff utilities
+export {
+  generateSyntheticDiffForNewFile,
+  appendUntrackedFileDiffs,
+  listAllFilesInDirectory,
+  generateDiffsForNonGitDirectory,
+  getGitRepositoryDiffs,
+} from './diff';