fix(sleep): reset ClaudeCliBackend.last_call_error at each call start

Follow-up to #126. ClaudeCliBackend recorded last_call_error on failure but
never reset it, so a failure followed by a successful call still reported the
stale error in persisted diagnostics — making a healthy run look failed. Both
_call() and attempt_with_tools() now clear it at the start, matching
CodexCliBackend.

Note: the "TimeoutExpired leaks the prompt into logs" concern raised in review
does NOT apply on current main — the prompt is passed via stdin (input=), not
argv (fixed by #98), so TimeoutExpired's str() only contains the argv (no
prompt). Verified empirically.

Adds a regression test asserting a prior failure is cleared on the next
success.

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

tests/test_sleep_engine.py

@@ -1148,6 +1148,26 @@ class TestClaudeCliBackendBare(unittest.TestCase):
         self.assertEqual(resp, "")
         self.assertIn("Claude CLI spawn failed", be.last_call_error)
 
+    def test_last_call_error_cleared_on_next_success(self):
+        """A prior failure must not make a later successful call look failed:
+        _call resets last_call_error at the start of each call."""
+        from unittest import mock
+        from skillopt_sleep.backend import ClaudeCliBackend
+        be = ClaudeCliBackend(claude_path="claude", timeout=3)
+        # First call fails to spawn -> error recorded.
+        with mock.patch("skillopt_sleep.backend.subprocess.run",
+                        side_effect=FileNotFoundError("no claude")):
+            self.assertEqual(be._call("p1"), "")
+        self.assertIn("Claude CLI spawn failed", be.last_call_error)
+        # Next call succeeds -> stale error must be cleared.
+
+        class _OK:
+            stdout = "answer"
+            stderr = ""
+        with mock.patch("skillopt_sleep.backend.subprocess.run", return_value=_OK()):
+            self.assertEqual(be._call("p2"), "answer")
+        self.assertEqual(be.last_call_error, "")
+