Fix task worktree path derivation to anchor on the main checkout

Why: cmd_start derived the worktree location from $PWD, so running task.sh start from a repository subdirectory (or inside a task worktree) placed the new worktree at the wrong spot — e.g. scripts-wt-<slug> nested inside the repo.
Evidence: bash -n clean; functional smoke of the patched script from both the main checkout scripts/ subdir and the repo root created ../Agent-System-Dev-wt-<slug> as a sibling of the main checkout (paths asserted; pre-fix subdir run would nest inside the repo); git rev-parse --git-common-dir resolves to the main checkout from both locations (relative ../.git vs absolute).
Impact: scripts/task.sh cmd_start only; from the main checkout root the derived path is identical to before; new worktrees now always land as siblings of the main checkout regardless of invocation directory.
Refs: .task/feature-task-start-rootfix.md

scripts/task.sh

@@ -63,10 +63,13 @@ cmd_start() {
   [ "$(git status --porcelain | { grep -v '^.. \.zcode/' || true; } | wc -l)" -eq 0 ] || die "dirty tree — resolve first (precheck verdict BUSY)"
   [ "$(git submodule status --recursive 2>/dev/null | { grep -cE '^\+|^U' || true; })" -eq 0 ] || die "submodule drift/conflict — resolve first (precheck verdict BUSY)"
   git merge-base --is-ancestor "origin/${TARGET_BRANCH}" HEAD || die "local ${TARGET_BRANCH} behind origin — pull --ff-only first"
-  local rmain slug wt
+  local rmain slug wt mainroot
   rmain=$(git rev-parse "origin/${TARGET_BRANCH}")
   slug=$(echo "$name" | tr '/' '-')
-  wt="$(dirname "$(pwd)")/$(basename "$(pwd)")-wt-${slug}"
+  # 落点须为主检出的平级目录：$PWD 推算在子目录（或任务 worktree 内）运行时会算错，
+  # 改用 git-common-dir 的父目录定位主检出（与 cmd_finish 同款惯用法）。
+  mainroot=$(dirname "$(cd "$(git rev-parse --git-common-dir)" && pwd)")
+  wt="$(dirname "$mainroot")/$(basename "$mainroot")-wt-${slug}"
   [ -e "$wt" ] && die "worktree path exists: $wt"
   git worktree add -q "$wt" -b "$name" "$rmain"
   (cd "$wt" && git submodule update --init --recursive >/dev/null 2>&1 || echo "[warn] submodule init incomplete in worktree")