fix: merge platform_overrides env with base env instead of replacing

platform_overrides.*.env was replacing the entire mcp_config.env object
instead of merging, causing base env vars (including ${user_config.*}
references) to be silently dropped when platform overrides defined any
env vars.

Fixes #217

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/shared/config.ts

@@ -117,7 +117,9 @@ export async function getMcpConfigForManifest(
 
       result.command = platformConfig.command || result.command;
       result.args = platformConfig.args || result.args;
-      result.env = platformConfig.env || result.env;
+      result.env = platformConfig.env
+        ? { ...result.env, ...platformConfig.env }
+        : result.env;
     }
   }