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/opencode-plugin/api-key.ts

@@ -1,6 +1,7 @@
 import {readFileSync} from "fs";
 import {homedir} from "os";
 import {join} from "path";
+import {mem0CliApiKey} from "../agent-plugin-core/typescript/src/credentials.ts";
 
 const PROFILE_FILES = [".zshrc", ".bashrc", ".zprofile", ".bash_profile", ".profile"];
 
@@ -26,5 +27,5 @@ export function resolveApiKey(env: NodeJS.ProcessEnv = process.env, homeDir = ho
     }
   }
 
-  return "";
+  return mem0CliApiKey(homeDir);
 }