fix: CLI stderr handler keyed on ownership, not presence

Greptile P2 on PR #252: the any-handler early return meant a foreign handler attached to the semble logger by an embedding application would suppress the CLI stderr handler, redirecting or discarding oversized-file warnings. Idempotence now keys on the CLI-owned handler type (_CliLogHandler), so repeated setup still adds exactly one handler while a preconfigured foreign logger no longer blocks stderr visibility.

Lore-id: f488e6e1
Constraint: CLI logging setup must be idempotent across repeated in-process invocations
Tested: 328 tests pass; ruff, ruff format, mypy clean
Tested: in-process check: with a foreign NullHandler attached, CLI handler installed exactly once across two setup calls
Related: c9380176
Confidence: high
Scope-risk: narrow
Reversibility: clean

Author: TommyC81

src/semble/cli.py

@@ -208,12 +208,16 @@ def _run_clear(clear_type: _CLEAR_CHOICE) -> None:
         _clear_orphans(cache_folder)
 
 
+class _CliLogHandler(logging.StreamHandler):
+    """stderr handler owned by the CLI; setup is idempotent on this type, not on foreign handlers."""
+
+
 def _configure_cli_logging() -> None:
     """Surface semble warnings (e.g. skipped oversized files) on stderr without touching the root logger."""
     package_logger = logging.getLogger("semble")
-    if package_logger.handlers:
+    if any(isinstance(handler, _CliLogHandler) for handler in package_logger.handlers):
         return
-    handler = logging.StreamHandler(sys.stderr)
+    handler = _CliLogHandler(sys.stderr)
     handler.setFormatter(logging.Formatter("%(levelname)s: %(message)s"))
     package_logger.addHandler(handler)
     if package_logger.level == logging.NOTSET: