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>

skillopt_sleep/backend.py

@@ -615,6 +615,9 @@ class ClaudeCliBackend(CliBackend):
         #   --exclude-dynamic-...     drop per-machine cwd/env/memory/git sections
         #   cwd=<clean temp>          no project CLAUDE.md
         import tempfile
+        # Reset per call so a prior failure does not make a later successful
+        # call look failed in persisted diagnostics (matches CodexCliBackend).
+        self.last_call_error = ""
         cmd = [self.claude_path, "-p", "--output-format", "text"]
         if os.environ.get("ANTHROPIC_API_KEY"):
             cmd.append("--bare")
@@ -657,6 +660,7 @@ class ClaudeCliBackend(CliBackend):
         # validated honestly: we detect the call from the shim's log, not from
         # a self-reported marker. Other tools are stubbed the same way.
         import tempfile, shutil, stat
+        self.last_call_error = ""
         work = tempfile.mkdtemp(prefix="skillopt_sleep_tools_")
         calllog = os.path.join(work, "_tool_calls.log")
         try: