fix(agent): make compaction settings dataclass compatible with Python 3.11+

Use field(default_factory=...) instead of a mutable-default-class instance, which Python 3.11+ dataclasses reject at import time.

core/agent/compaction.py

@@ -365,7 +365,11 @@ class CompactionPreparation:
     tokens_before: int = 0
     previous_summary: Optional[str] = None
     file_ops: FileOperations = field(default_factory=FileOperations)
-    settings: CompactionSettings = DEFAULT_COMPACTION_SETTINGS
+    # 注意（P2-04 跨平台修复）：Python 3.11+ 的 dataclass 禁止不可哈希默认值
+    # （eq dataclass 实例 __hash__=None）；Ubuntu 24.04 自带 3.12，
+    # 直接写 `= DEFAULT_COMPACTION_SETTINGS` 在 3.11+ 会 ValueError。
+    # default_factory 返回同一共享实例，与 3.10 行为完全一致。
+    settings: CompactionSettings = field(default_factory=lambda: DEFAULT_COMPACTION_SETTINGS)
 
 
 def _compaction_diag(line: str) -> None: