Avoid new commits when the task already contains the integration baseline

Why: repeated task-sync calls changed HEAD and invalidated CI even when main had not advanced.

Evidence: SELF red-green regression and 58 task lifecycle tests recorded through make evidence in the task card.

Impact: sync still fetches and validates components, creates records only for an actual merge, and reports the complete final source SHA.

Refs: U1; policy/merge-queue.md; .task/feature-merge-queue-policy-draft.md

scripts/task.sh

@@ -278,18 +278,26 @@ cmd_sync() {
   need_repo
   fetch_target
   check_worktree before-sync
-  git merge --no-edit "origin/${TARGET_BRANCH}" \
-    -m "Merge authoritative ${TARGET_BRANCH} into the task branch" \
-    -m "Why: include the current integration baseline before verification." \
-    -m "Evidence: fetched origin/${TARGET_BRANCH}=$(git rev-parse "origin/${TARGET_BRANCH}")." \
-    -m "Impact: integrate upstream changes without rewriting task history." \
-    -m "Refs: task-sync"
-  git merge-base --is-ancestor "origin/${TARGET_BRANCH}" HEAD || die "sync incomplete"
+  local base head card branch slug pc changed=0 ancestry_status
+  base=$(git rev-parse "origin/${TARGET_BRANCH}")
+  if git merge-base --is-ancestor "$base" HEAD; then
+    :
+  else
+    ancestry_status=$?
+    [ "$ancestry_status" -eq 1 ] || die "cannot verify target ancestry"
+    git merge --no-edit "$base" \
+      -m "Merge authoritative ${TARGET_BRANCH} into the task branch" \
+      -m "Why: include the current integration baseline before verification." \
+      -m "Evidence: fetched origin/${TARGET_BRANCH}=${base}." \
+      -m "Impact: integrate upstream changes without rewriting task history." \
+      -m "Refs: task-sync"
+    changed=1
+  fi
+  git merge-base --is-ancestor "$base" HEAD || die "sync incomplete"
   check_worktree before-update
   check_worktree materialize
   check_worktree after-sync
-  local base head card branch slug pc
-  base=$(git rev-parse "origin/${TARGET_BRANCH}"); head=$(git rev-parse HEAD)
+  head=$(git rev-parse HEAD)
   branch=$(git rev-parse --abbrev-ref HEAD)
   slug=$(echo "$branch" | tr '/' '-')
   card=".task/${slug}.md"
@@ -304,18 +312,21 @@ cmd_sync() {
     done
   fi
   if [ -n "$card" ] && [ -f "$card" ]; then
-    printf '| 同步前验收 | %s | %s | %s |\n' "$base" "$head" "$(date -u +%FT%TZ)" >> "$card"
-    git add "$card"
-    git commit -q -m "Record the sync checkpoint" \
-      -m "Why: bind the task validation checkpoint to the latest integration baseline." \
-      -m "Evidence: SELF recursive component checks; sync_base=${base}; candidate=${head}." \
-      -m "Impact: task metadata only." -m "Refs: ${card}"
+    if [ "$changed" -eq 1 ]; then
+      printf '| 同步（记录前对象） | %s | %s | %s |\n' "$base" "$head" "$(date -u +%FT%TZ)" >> "$card"
+      git add "$card"
+      git commit -q -m "Record the sync checkpoint" \
+        -m "Why: bind the task validation checkpoint to the integration baseline." \
+        -m "Evidence: SELF recursive component checks; sync_base=${base}; before_record=${head}." \
+        -m "Impact: task metadata only." -m "Refs: ${card}" -- "$card"
+    fi
     # JSON receipts carry before/after state; older table rows remain readable.
     python3 "${SCRIPT_DIR}/check-evidence.py" "$card"
   else
     echo "[warn] 找不到分支 ${branch} 的任务卡，同步记录未写入" >&2
   fi
-  echo "[synced] sync_base=$(echo "$base" | cut -c1-10) tested_head=$(echo "$head" | cut -c1-10) checked_at=$(date -u +%FT%TZ)"
+  head=$(git rev-parse HEAD)
+  echo "[synced] sync_base=${base} tested_head=${head} checked_at=$(date -u +%FT%TZ) changed=${changed}"
   echo "[next] 跑正式验收（make test 等），证据写入任务卡"
 }