fix(skillopt-sleep): surface Claude CLI spawn failures instead of silent zero scores (#126)

* fix(skillopt-sleep): surface Claude CLI spawn failures instead of silent zero scores

In _call and attempt_with_tools, the bare 'except Exception: return ""'
swallows FileNotFoundError (e.g. bare 'claude' on Windows with npm .cmd
shim) and any other spawn failure, returning an empty string that the
trainer treats as a legitimate model response that scores 0.0 everywhere.

Now the exception is caught explicitly: last_call_error is set, a
warning is logged, and the empty-string return is preserved for
backward compatibility of the control flow. This mirrors the pattern
from #92 (codex backend) which fixed the same class of 'dead CLI
masquerades as nothing to learn' bug.

Issue: #121

* test(skillopt-sleep): add tests verifying Claude CLI spawn failures are surfaced

Add two tests to TestClaudeCliBackendBare:
- test_spawn_failure_sets_last_call_error: _call sets last_call_error
  and returns '' when subprocess.run raises FileNotFoundError.
- test_attempt_tools_spawn_failure_sets_last_call_error: same for
  attempt_with_tools.

These prove the fix from the parent commit (surface spawn failures
instead of silently scoring 0) and guard against regressions.

tests/test_sleep_engine.py

@@ -1118,6 +1118,36 @@ class TestClaudeCliBackendBare(unittest.TestCase):
         # But it's also recorded for detection
         self.assertIn("Not logged in", getattr(be, "last_call_error", ""))
 
+    def test_spawn_failure_sets_last_call_error(self):
+        """When subprocess.run raises FileNotFoundError, _call must set
+        last_call_error and log a warning instead of silently returning ''."""
+        from skillopt_sleep.backend import ClaudeCliBackend
+        be = ClaudeCliBackend(
+            claude_path="/nonexistent/claude-binary",
+            timeout=3,
+        )
+        result = be._call("test prompt")
+        self.assertEqual(result, "")
+        self.assertIn("Claude CLI spawn failed", be.last_call_error)
+
+    def test_attempt_tools_spawn_failure_sets_last_call_error(self):
+        """When subprocess.run raises in attempt_with_tools, last_call_error
+        must be set and a warning logged."""
+        from skillopt_sleep.backend import ClaudeCliBackend
+        from skillopt_sleep.types import TaskRecord
+        be = ClaudeCliBackend(
+            claude_path="/nonexistent/claude-binary",
+            timeout=3,
+        )
+        task = TaskRecord(
+            id="t1", project="/p", intent="test",
+            reference="ref", reference_kind="exact",
+            tags=[], split="train",
+        )
+        resp, called = be.attempt_with_tools(task, "", "", tools=["search"])
+        self.assertEqual(resp, "")
+        self.assertIn("Claude CLI spawn failed", be.last_call_error)
+