fix(mem0): merge env vars with mem0.json instead of either/or

When mem0.json exists but is missing the api_key (e.g. after running
`hermes memory setup`), the plugin reports "not available" even though
MEM0_API_KEY is set in .env.  This happens because _load_config()
returns the JSON file contents verbatim, never falling back to env vars.

Use env vars as the base config and let mem0.json override individual
keys on top, so both config sources work together.

Fixes: mem0 plugin shows "not available" despite valid MEM0_API_KEY in .env

plugins/memory/mem0/__init__.py

@@ -38,17 +38,15 @@ _BREAKER_COOLDOWN_SECS = 120
 # ---------------------------------------------------------------------------
 
 def _load_config() -> dict:
-    """Load config from $HERMES_HOME/mem0.json or env vars."""
+    """Load config from env vars, with $HERMES_HOME/mem0.json overrides.
+
+    Environment variables provide defaults; mem0.json (if present) overrides
+    individual keys.  This avoids a silent failure when the JSON file exists
+    but is missing fields like ``api_key`` that the user set in ``.env``.
+    """
     from hermes_constants import get_hermes_home
-    config_path = get_hermes_home() / "mem0.json"
 
-    if config_path.exists():
-        try:
-            return json.loads(config_path.read_text(encoding="utf-8"))
-        except Exception:
-            pass
-
-    return {
+    config = {
         "api_key": os.environ.get("MEM0_API_KEY", ""),
         "user_id": os.environ.get("MEM0_USER_ID", "hermes-user"),
         "agent_id": os.environ.get("MEM0_AGENT_ID", "hermes"),
@@ -56,6 +54,16 @@ def _load_config() -> dict:
         "keyword_search": False,
     }
 
+    config_path = get_hermes_home() / "mem0.json"
+    if config_path.exists():
+        try:
+            file_cfg = json.loads(config_path.read_text(encoding="utf-8"))
+            config.update({k: v for k, v in file_cfg.items() if v})
+        except Exception:
+            pass
+
+    return config
+
 
 # ---------------------------------------------------------------------------
 # Tool schemas