fix(mcp): restrict env var expansion in MCP configs

Block sensitive env var interpolation in MCP config expansion so repo and plugin MCP definitions cannot exfiltrate secrets by default.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/claude-code-mcp-loader/index.ts

@@ -9,3 +9,4 @@ export * from "./types"
 export * from "./loader"
 export * from "./transformer"
 export * from "./env-expander"
+export * from "./configure-allowed-env-vars"