fix(memory): read/write .env as UTF-8 in mem0 and hindsight setup

The mem0 and hindsight memory-provider setup routines round-trip the
user's ~/.hermes/.env: they read existing lines, update the keys they
manage, and rewrite the whole file preserving every other line verbatim.
Both used env_path.read_text() / write_text() with no encoding.

read_text()/write_text() with no encoding fall back to the system locale
(cp1252/GBK on Windows), so on a non-UTF-8 host the preserved lines get
mangled or the call crashes on any non-ASCII value, and — because the
reader never strips a BOM — a Notepad-edited .env makes the first key
fail the in-place match and get duplicated instead of updated.

Match the canonical .env readers in hermes_cli/config.py: read with
encoding='utf-8-sig' (BOM-tolerant) and write with encoding='utf-8'.
mem0/_setup.py already pins utf-8 for mem0.json, so this just aligns the
.env path in the same file. Fixes both memory plugins in one class fix.

Adds regression tests: a BOM'd .env updates the first key in place
(locale-independent, fails without the fix) and non-ASCII existing lines
survive the round-trip.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugins/memory/mem0/_setup.py

@@ -193,7 +193,14 @@ def _write_env(env_path: Path, env_writes: dict[str, str]) -> None:
     env_path.parent.mkdir(parents=True, exist_ok=True)
     existing_lines: list[str] = []
     if env_path.exists():
-        existing_lines = env_path.read_text().splitlines()
+        # Read as UTF-8 (BOM-tolerant), matching the canonical .env readers in
+        # hermes_cli/config.py. read_text() with no encoding falls back to the
+        # system locale (cp1252/GBK on Windows): it mangles or crashes on
+        # non-ASCII values while copying existing lines through, and a BOM'd
+        # first line would fail the key match and get duplicated.
+        existing_lines = env_path.read_text(
+            encoding="utf-8-sig"
+        ).splitlines()
 
     updated_keys: set[str] = set()
     new_lines: list[str] = []
@@ -208,7 +215,7 @@ def _write_env(env_path: Path, env_writes: dict[str, str]) -> None:
         if k not in updated_keys:
             new_lines.append(f"{k}={v}")
 
-    env_path.write_text("\n".join(new_lines) + "\n")
+    env_path.write_text("\n".join(new_lines) + "\n", encoding="utf-8")
 
 
 def _save_mem0_json(hermes_home: str, data: dict) -> None:

tests/plugins/memory/test_mem0_setup.py

@@ -181,6 +181,26 @@ class TestWriteEnv:
         assert "OTHER=keep" in content
         assert "old" not in content
 
+    def test_preserves_non_ascii_existing_lines(self, tmp_path):
+        """Existing non-ASCII .env content must survive the read-modify-write
+        as UTF-8 (the locale codec would crash/mangle it on Windows)."""
+        env_path = tmp_path / ".env"
+        env_path.write_bytes("PROXY_NOTE=café-zürich-完了\n".encode("utf-8"))
+        _write_env(env_path, {"OPENAI_API_KEY": "sk-test"})
+        content = env_path.read_text(encoding="utf-8")
+        assert "PROXY_NOTE=café-zürich-完了" in content
+        assert "OPENAI_API_KEY=sk-test" in content
+
+    def test_updates_first_key_with_bom(self, tmp_path):
+        """A Notepad-edited .env carries a BOM; the first key must still be
+        matched/updated in place, not duplicated."""
+        env_path = tmp_path / ".env"
+        env_path.write_bytes("OPENAI_API_KEY=old\n".encode("utf-8"))
+        _write_env(env_path, {"OPENAI_API_KEY": "new"})
+        content = env_path.read_text(encoding="utf-8")
+        assert content.count("OPENAI_API_KEY=") == 1
+        assert "OPENAI_API_KEY=new" in content
+
 
 class TestPostSetup: