Move the sync checkpoint onto this task card

Why: task-sync appends its checkpoint to the first file in .task, which was another task's card.
Evidence: The 2026-10-01T10:39:24Z row now sits in feature-base-extension-roadmap.md. The other card matches its previous content.
Impact: The recorded sync base is origin/main 26dfea7ed6 and the tested head is the merge 6d496139b4.
Refs: .task/feature-base-extension-roadmap.md
Co-authored-by: Cursor <cursoragent@cursor.com>

.task/feature-agent-professional-deliverables.md

@@ -28,4 +28,3 @@
 - 影响与剩余项：Backend PR #38 已合入（merge 2079b2dfa851bd4cbe8f399a015126c842c441ec）。本分支推进 gitlink 到该提交。样例文档仍留在主检出 .tmp，不入库。工件卡片上的“设为默认模板”按钮仍不在本次范围。
 | 同步前验收 | 6f7f4a0e7858c70261d7bbeb179f5eb59303f956 | 2e8b9face3542844319f4a545768b6cf04c126a5 | 2026-10-01T08:19:31Z |
 | 同步前验收 | 6f7f4a0e7858c70261d7bbeb179f5eb59303f956 | f41ad95b1c5c9d2c415a12a64ac1f1551cd7b6af | 2026-10-01T08:20:22Z |
-| 同步前验收 | 26dfea7ed62575097f14fedf1b7713a77dedb66f | 6d496139b45bbe0d32818e56ed6c5fbba649d777 | 2026-10-01T10:39:24Z |

.task/feature-base-extension-roadmap.md

@@ -18,6 +18,7 @@
 | 节点 | sync_base_sha | tested_head_sha | checked_at |
 |---|---|---|---|
 | 启动 | f8fefa0f888e8f5fd5207c9e97099e59935ff69b | f8fefa0f888e8f5fd5207c9e97099e59935ff69b | 2026-10-01T10:10:35Z |
+| 同步前验收 | 26dfea7ed62575097f14fedf1b7713a77dedb66f | 6d496139b45bbe0d32818e56ed6c5fbba649d777 | 2026-10-01T10:39:24Z |
 
 ## 结果与验证（完成后填写）