feat: add git branch support for worktree creation

Add optional branch configuration to specify which git branch to use
for worktree creation.

Features:
- New branch config option to specify base branch (e.g., main, develop)
- Each worktree creates a unique branch based on the specified branch
  (format: branch-run-id)
- If branch is not specified, uses the repository default branch
- Falls back gracefully when branch already exists

Changes:
- Add branch field to RunConfig dataclass
- Update GitWorktreeManager.create() to accept branch parameter
- Create unique branches for each worktree to avoid conflicts
- Update load_config() to parse branch from config
- Update config.example.json with branch option
- Update README.md documentation

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

integration-tests/concurrent-runner/config.example.json

@@ -2,6 +2,7 @@
   "concurrency": 3,
   "yolo": true,
   "source_repo": ".",
+  "branch": "main",
   "worktree_base": "~/.qwen/worktrees",
   "outputs_dir": "./outputs",
   "results_file": "./results.json",