feat: worktree 隔离方案 - 多 agent 独立工作目录 - Closes #102

启动 agent 后自动创建 ~/.gitea/worktrees/<user>/ 隔离目录，
多个 agent 可同时修改不同文件、不同分支互不干扰。

- _common.sh: 新增 setup_worktree/cleanup_worktree 函数
- start_dev_agent.sh: 启动时自动切 worktree
- start_qe_agent.sh: 同上
- DEV_AGENT.md/QE_AGENT.md: 启动行为增加 worktree 检查步骤

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/_common.sh

@@ -6,7 +6,8 @@ set -eu
 
 # ── Resolve paths ──────────────────────────────────────────────────────────────
 _COMMON_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
-PROJECT_DIR="${PROJECT_DIR:-$(cd "$_COMMON_DIR/.." && pwd)}"
+_MAIN_REPO_DIR="$(cd "$_COMMON_DIR/.." && pwd)"
+PROJECT_DIR="${PROJECT_DIR:-$_MAIN_REPO_DIR}"
 
 # ── Load Gitea configuration ────────────────────────────────────────────────────
 # Primary: ~/.gitea/config.yaml (requires GITEA_USER)
@@ -18,6 +19,41 @@ if ! eval "$(python "$_COMMON_DIR/_get_gitea_config.py" 2>/dev/null)"; then
     fi
 fi
 
+# ── Worktree isolation ─────────────────────────────────────────────────────────
+GITEA_WORKTREE_DIR="${GITEA_WORKTREE_DIR:-$HOME/.gitea/worktrees}"
+
+setup_worktree() {
+    local user="$1"
+    local worktree="$GITEA_WORKTREE_DIR/$user"
+
+    # Already inside a worktree we created — reuse it.
+    if [ -f "$worktree/.gitea-worktree" ]; then
+        echo "Using existing worktree: $worktree"
+        PROJECT_DIR="$worktree"
+        cd "$PROJECT_DIR"
+        return 0
+    fi
+
+    local branch="agent/${user}/$(date +%Y%m%d-%H%M%S)"
+    echo "Creating worktree: $worktree (branch: $branch)"
+    mkdir -p "$GITEA_WORKTREE_DIR"
+    git -C "$_MAIN_REPO_DIR" worktree add -b "$branch" "$worktree" origin/main
+    touch "$worktree/.gitea-worktree"
+    PROJECT_DIR="$worktree"
+    cd "$PROJECT_DIR"
+}
+
+cleanup_worktree() {
+    local user="$1"
+    local worktree="$GITEA_WORKTREE_DIR/$user"
+    if [ -d "$worktree" ]; then
+        rm -f "$worktree/.gitea-worktree"
+        echo "Cleaning up worktree: $worktree"
+        git -C "$_MAIN_REPO_DIR" worktree remove "$worktree" 2>/dev/null || true
+        rm -rf "$worktree" 2>/dev/null || true
+    fi
+}
+
 # ── Validate required environment ──────────────────────────────────────────────
 require_token() {
     if [ -z "${GITEA_API_TOKEN:-}" ]; then