worktree_sync(fix[sync_worktree]) Guard against empty plan entries list

why: plan_worktree_sync should always return at least one entry for a single
config input, but an internal bug could cause an empty list. An IndexError
with no context is harder to debug than an explicit ERROR entry.

what:
- Add guard before entries[0] access in sync_worktree
- Return ERROR entry with descriptive message when plan is empty
- Add test mocking empty plan return to verify ERROR instead of crash

Author: tony

src/vcspull/_internal/worktree_sync.py

@@ -655,6 +655,15 @@ def sync_worktree(
     """
     # Plan the operation
     entries = plan_worktree_sync(repo_path, [wt_config], workspace_root)
+    if not entries:
+        log.warning("plan_worktree_sync returned empty list for %s", wt_config)
+        return WorktreePlanEntry(
+            worktree_path=_resolve_worktree_path(wt_config, workspace_root),
+            ref_type="unknown",
+            ref_value="unknown",
+            action=WorktreeAction.ERROR,
+            error="internal: planning produced no entries",
+        )
     entry = entries[0]
 
     if dry_run or entry.action in (WorktreeAction.ERROR, WorktreeAction.BLOCKED):

tests/test_worktree.py

@@ -3389,3 +3389,31 @@ def mock_run(*args: t.Any, **kwargs: t.Any) -> t.Any:
     # Creation should succeed despite lock failure
     assert entry.action == WorktreeAction.CREATE
     assert worktree_path.exists()
+
+
+def test_sync_worktree_empty_plan_returns_error(
+    tmp_path: pathlib.Path,
+    mocker: MockerFixture,
+) -> None:
+    """Test sync_worktree returns ERROR entry when plan returns empty list.
+
+    Defense-in-depth: plan_worktree_sync should always return at least one
+    entry, but if it doesn't, sync_worktree should return an ERROR entry
+    instead of crashing with IndexError.
+    """
+    # Mock plan_worktree_sync to return empty list
+    mocker.patch(
+        "vcspull._internal.worktree_sync.plan_worktree_sync",
+        return_value=[],
+    )
+
+    wt_config: WorktreeConfigDict = {
+        "dir": "../empty-plan-wt",
+        "tag": "v1.0.0",
+    }
+
+    entry = sync_worktree(tmp_path, wt_config, tmp_path)
+
+    assert entry.action == WorktreeAction.ERROR
+    assert entry.error is not None
+    assert "no entries" in entry.error