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.

skillopt_sleep/backend.py

@@ -634,7 +634,12 @@ class ClaudeCliBackend(CliBackend):
                 cmd, capture_output=True, creationflags=_NO_WINDOW, text=True, timeout=self.timeout, cwd=clean_cwd,
                 input=prompt,
             )
-        except Exception:
+        except Exception as exc:
+            import logging
+            self.last_call_error = f"Claude CLI spawn failed: {exc}"
+            logging.getLogger("skillopt_sleep").warning(
+                "Claude CLI could not be executed: %s", exc,
+            )
             return ""
         finally:
             try:
@@ -698,7 +703,12 @@ class ClaudeCliBackend(CliBackend):
                 )
                 resp = (proc.stdout or "").strip()
                 self._detect_cli_error(resp, proc.stderr or "")
-            except Exception:
+            except Exception as exc:
+                import logging
+                self.last_call_error = f"Claude CLI spawn failed: {exc}"
+                logging.getLogger("skillopt_sleep").warning(
+                    "Claude CLI could not be executed: %s", exc,
+                )
                 resp = ""
             self._tokens += len(prompt) // 4 + len(resp) // 4
             called: List[str] = []