Fix mcp-server-utils package exports

[xianzuyang9-blip]

Summary

• add a root barrel for @gleanwork/mcp-server-utils so the existing main entrypoint is importable through the package exports map
• remove stale exports for auth and tools modules that are not built or shipped by this package

Fixes #360.

Verification

• corepack pnpm --filter @gleanwork/mcp-test-utils build
• corepack pnpm --filter @gleanwork/mcp-server-utils build
• corepack pnpm --filter @gleanwork/mcp-server-utils test (67 tests passed)
• npm pack --ignore-scripts after build, clean consumer import of root/config/errors/util/logger succeeded and package exports only listed shipped entrypoints

[rwjblue-glean]

Thanks for fixing this!