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/plugin-handlers/config-handler.test.ts

@@ -57,6 +57,7 @@ beforeEach(() => {
   spyOn(agentLoader, "loadProjectAgents" as any).mockReturnValue({})
 
   spyOn(mcpLoader, "loadMcpConfigs" as any).mockResolvedValue({ servers: {} })
+  spyOn(mcpLoader, "setAdditionalAllowedMcpEnvVars").mockImplementation(() => {})
 
   spyOn(pluginLoader, "loadAllPluginComponents" as any).mockResolvedValue({
     commands: {},
@@ -103,6 +104,7 @@ afterEach(() => {
   ;(agentLoader.loadUserAgents as any)?.mockRestore?.()
   ;(agentLoader.loadProjectAgents as any)?.mockRestore?.()
   ;(mcpLoader.loadMcpConfigs as any)?.mockRestore?.()
+  ;(mcpLoader.setAdditionalAllowedMcpEnvVars as any)?.mockRestore?.()
   ;(pluginLoader.loadAllPluginComponents as any)?.mockRestore?.()
   ;(mcpModule.createBuiltinMcps as any)?.mockRestore?.()
   ;(shared.log as any)?.mockRestore?.()
@@ -173,6 +175,36 @@ describe("Sisyphus-Junior model inheritance", () => {
   })
 })
 
+describe("MCP env allowlist initialization", () => {
+  test("sets the configured MCP env allowlist before plugin loading", async () => {
+    // given
+    const pluginConfig = createPluginConfig({
+      mcp_env_allowlist: ["CUSTOM_API_KEY", "CUSTOM_AUTH_TOKEN"],
+    })
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // when
+    await handler(config)
+
+    // then
+    expect(mcpLoader.setAdditionalAllowedMcpEnvVars).toHaveBeenCalledWith([
+      "CUSTOM_API_KEY",
+      "CUSTOM_AUTH_TOKEN",
+    ])
+  })
+})
+
 describe("Plan agent demote behavior", () => {
   test("orders core agents as sisyphus -> hephaestus -> prometheus -> atlas", async () => {
     // #given