fix(plugins): read the mem0 CLI key and ignore unexpanded host placeholders

The Python plugin core and the opencode, pi and deepseek plugins now fall back to the key `mem0 init` saves in ~/.mem0/config.json, so a configured CLI is enough to authenticate. The Python core also stops using or caching a literal ${api_key} left behind when a host (Cursor) does not expand its plugin variables, which is what kept asking for auth after the key was set.

Fixes #7346

integrations/claude-code-plugin/tests/test_telemetry.py

@@ -24,6 +24,8 @@ def isolated_env(tmp_path, monkeypatch):
     monkeypatch.delenv("CLAUDE_PLUGIN_OPTION_API_KEY", raising=False)
     monkeypatch.delenv("CLAUDE_PLUGIN_OPTION_MEM0_API_KEY", raising=False)
     monkeypatch.delenv("MEM0_API_URL", raising=False)
+    monkeypatch.setenv("HOME", str(tmp_path / "home"))
+    monkeypatch.setenv("USERPROFILE", str(tmp_path / "home"))
     return tmp_path