fix(sleep): handoff no-task cleanup + corrupt tasks.json fallback

Follow-up to #125. Two resumability defects in the handoff driver:

1. A no-task night left digests.json (often []) pinned, so every later run
   reloaded the empty pin and never called harvest_for_config again. Remove the
   pin on terminal no-task completion so the next run re-harvests.

2. An interrupted/corrupt tasks.json snapshot raised an uncaught
   JSONDecodeError in _run_handoff. Wrap load_tasks_file in try/except and
   re-mine instead of crashing.

Adds regression tests for both.

Co-Authored-By: Claude <noreply@anthropic.com>

skillopt_sleep/__main__.py

@@ -311,6 +311,13 @@ def _handoff_mine_and_pin(cfg, args, backend, snapshot: str, dry: bool):
             # branch, or every later run re-scans the same stale window.
             state.set_last_harvest(project, started)
             state.save()
+            # Drop the pinned (often empty) digests so the NEXT run re-harvests
+            # instead of reloading a []-pin and never calling harvest again.
+            try:
+                if os.path.exists(digests_path):
+                    os.remove(digests_path)
+            except OSError:
+                pass
         return 0, None
     payload = make_tasks_payload(
         tasks,
@@ -342,13 +349,21 @@ def _run_handoff(cfg, args, *, seed_tasks, task_meta: Dict[str, Any], dry: bool)
     tasks = seed_tasks
     if tasks is None:
         snapshot = os.path.join(hdir, "tasks.json")
+        tasks = None
         if os.path.exists(snapshot):
+            try:
                 tasks, _meta = load_tasks_file(
                     snapshot,
                     holdout_fraction=cfg.get("holdout_fraction", 0.34),
                     seed=cfg.get("seed", 42),
                 )
-        else:
+            except (ValueError, json.JSONDecodeError, OSError):
+                # Interrupted/corrupt pin from an earlier round: re-mine instead
+                # of crashing the run.
+                print("[sleep] handoff: tasks.json unreadable — re-mining",
+                      file=sys.stderr)
+                tasks = None
+        if tasks is None:
             rc, tasks = _handoff_mine_and_pin(cfg, args, backend, snapshot, dry)
             if tasks is None:
                 return rc

tests/test_handoff_backend.py

@@ -215,6 +215,45 @@ class TestHandoffCli(unittest.TestCase):
             state = SleepState.load(cfg.state_path)
             self.assertIsNotNone(state.last_harvest_for(proj))
 
+    def test_no_tasks_removes_digests_pin_so_next_run_reharvests(self):
+        """After a no-task night the (often empty) digests.json pin must be
+        removed, or every later run reloads a []-pin and never harvests again."""
+        from skillopt_sleep.__main__ import main
+
+        with tempfile.TemporaryDirectory() as proj, \
+                tempfile.TemporaryDirectory() as home:
+            hdir = os.path.join(proj, ".skillopt-sleep-handoff")
+            os.makedirs(hdir, exist_ok=True)
+            digests_path = os.path.join(hdir, "digests.json")
+            with open(digests_path, "w", encoding="utf-8") as f:
+                json.dump([], f)
+            rc = main([
+                "run", "--backend", "handoff",
+                "--project", proj,
+                "--claude-home", os.path.join(home, ".claude"),
+            ])
+            self.assertEqual(rc, 0)
+            # the empty pin must be gone so the next run re-harvests
+            self.assertFalse(os.path.exists(digests_path))
+
+    def test_corrupt_tasks_pin_falls_back_to_remine(self):
+        """An interrupted/corrupt tasks.json snapshot must not crash the run."""
+        from skillopt_sleep.__main__ import main
+
+        with tempfile.TemporaryDirectory() as proj, \
+                tempfile.TemporaryDirectory() as home:
+            hdir = os.path.join(proj, ".skillopt-sleep-handoff")
+            os.makedirs(hdir, exist_ok=True)
+            with open(os.path.join(hdir, "tasks.json"), "w", encoding="utf-8") as f:
+                f.write('{"tasks": [ truncated...')
+            rc = main([
+                "run", "--backend", "handoff",
+                "--project", proj,
+                "--claude-home", os.path.join(home, ".claude"),
+            ])
+            # corrupt snapshot -> re-mine -> no tasks -> 0 (not a crash)
+            self.assertEqual(rc, 0)
+
 
 if __name__ == "__main__":
     unittest.main()