fix(agent-wrapper): stop assuming POSIX-only calls and path styles

Windows runs hit three platform assumptions in the agent wrappers:

- `CodexAgentWrapper._effective_config_snapshot()` called `os.fchmod`,
  which only exists on POSIX. Building the MCP config snapshot therefore
  raised `AttributeError` on Windows and the Codex wrapper could not
  start. The call is now guarded by a capability check
  (`hasattr(os, "fchmod")`), which also keeps the guard testable on
  POSIX hosts.
- `CcFileSessionStore.list_subkeys()` rendered subpaths with `str()`,
  which yields backslashes on Windows. The session-store contract - and
  the SDK conformance check shipped with `claude_agent_sdk` - expects
  POSIX subpaths such as `subagents/agent-2`. Render them with
  `as_posix()`.
- `AutoMemoryStep` wrote session JSONL with the platform default newline
  translation, so Windows stored CRLF even though the writer appends
  `"\n"`. Pass `newline=""` on the append and rewrite paths so one
  message stays one LF-terminated line on every platform.

Add a regression test that pins the snapshot path while `os.fchmod` is
unavailable, so the guard is covered on POSIX CI too.

reme/components/agent_wrapper/cc_session_store.py

@@ -104,7 +104,7 @@ class CcFileSessionStore:
         if not session_dir.is_dir():
             return []
         return [
-            str(path.relative_to(session_dir).with_suffix(""))
+            path.relative_to(session_dir).with_suffix("").as_posix()
             for path in session_dir.rglob("*.jsonl")
             if path.is_file()
         ]

reme/components/agent_wrapper/codex_agent_wrapper.py

@@ -175,7 +175,8 @@ class CodexAgentWrapper(BaseAgentWrapper):
         snapshot_dir.mkdir(parents=True, exist_ok=True)
         fd, raw_path = tempfile.mkstemp(prefix="config-", suffix=".json", dir=snapshot_dir)
         try:
-            os.fchmod(fd, 0o600)
+            if hasattr(os, "fchmod"):
+                os.fchmod(fd, 0o600)
             with os.fdopen(fd, "w", encoding="utf-8") as stream:
                 json.dump(self.app_context.app_config.model_dump(mode="json"), stream)
         except BaseException:

reme/steps/evolve/auto_memory.py

@@ -331,7 +331,7 @@ class AutoMemoryStep(BaseStep):
             if can_append:
                 new_msgs = merged[len(existing) :]
                 if new_msgs:
-                    async with aiofiles.open(path, "a", encoding="utf-8") as f:
+                    async with aiofiles.open(path, "a", encoding="utf-8", newline="") as f:
                         for msg in new_msgs:
                             await f.write(_sanitize_msg_for_save(msg).model_dump_json() + "\n")
                     self.logger.info(
@@ -344,7 +344,7 @@ class AutoMemoryStep(BaseStep):
                         f"existing={len(existing)} total={len(merged)}",
                     )
             else:
-                async with aiofiles.open(path, "w", encoding="utf-8") as f:
+                async with aiofiles.open(path, "w", encoding="utf-8", newline="") as f:
                     for msg in merged:
                         await f.write(_sanitize_msg_for_save(msg).model_dump_json() + "\n")
                 self.logger.info(

tests/unit/test_codex_agent_wrapper.py

@@ -5,6 +5,7 @@
 import asyncio
 from dataclasses import dataclass
 import json
+import os
 from pathlib import Path
 import stat
 import sys
@@ -575,6 +576,18 @@ def test_effective_mcp_config_snapshot_is_private_and_removed_on_close(tmp_path)
     assert not snapshot.exists()
 
 
+def test_effective_mcp_config_snapshot_needs_no_fchmod(tmp_path, monkeypatch):
+    """A platform without ``os.fchmod`` (Windows) still gets a usable snapshot."""
+    monkeypatch.delattr(os, "fchmod", raising=False)
+
+    wrapper, _job = _wrapper(tmp_path)
+    config = wrapper._mcp_server_config({"job_tools": ["search"]})
+    snapshot = Path(config["args"][config["args"].index("--config") + 1])
+
+    assert snapshot.exists()
+    assert json.loads(snapshot.read_text(encoding="utf-8"))["workspace_dir"] == str(tmp_path)
+
+
 @pytest.mark.asyncio
 async def test_effective_snapshot_exposes_parent_only_custom_job(tmp_path):
     from fastmcp import Client