Separate local task recovery from delivery readiness

Why: an existing authorized task should not merge a moving target merely to resume local work, while new task startup and delivery remain strict.

Evidence: SELF red-green recovery and environment regressions; task, recovery, evidence and local-mode write-guard suites passed through make evidence.

Impact: add explicit LOCAL_READY and task-resume, validate worktree/card/baseline and receipt purpose, preserve schema 1 and prevent local receipts from starting or releasing tasks.

Refs: U2; policy/merge-queue.md; .task/feature-merge-queue-policy-draft.md

tests/test_task.py

@@ -26,7 +26,8 @@ class TaskLifecycleTests(unittest.TestCase):
                          "GIT_AUTHOR_NAME": "Lifecycle Test", "GIT_AUTHOR_EMAIL": "test@example.invalid",
                          "GIT_COMMITTER_NAME": "Lifecycle Test", "GIT_COMMITTER_EMAIL": "test@example.invalid"})
         for key in ("GIT_DIR", "GIT_WORK_TREE", "GIT_INDEX_FILE", "GITEA_TOKEN", "PRECHECK_RECEIPT",
-                    "TASK_PARENT_BRANCH", "TASK_PARENT_PR", "FORCE_ABANDON", "TASK_TARGET_BRANCH"):
+                    "TASK_PARENT_BRANCH", "TASK_PARENT_PR", "FORCE_ABANDON", "TASK_TARGET_BRANCH",
+                    "TASK_WORK_MODE", "VALIDATE_RECEIPT_PURPOSE"):
             self.env.pop(key, None)
         self.git(self.root, "init", "--bare", "--initial-branch=main", str(self.remote))
         # receive-pack 默认在后台跑 gc --auto，与 TemporaryDirectory 清理竞态