Fix incorrect information about script-level variables

The previous version incorrectly stated that script-level variables
could be used to return data from psake to the calling script. This
is wrong because psake executes the build file in its own scope.

Key corrections:
- Removed script-level variables as primary recommendation
- Made output files (JSON/YAML) the #1 recommended approach
- Added clear anti-pattern section explaining why script variables
  don't work outside psake invocations
- Distinguished between inter-task communication (script vars work)
  and returning data from psake (must use files)
- Added direct answer to original question at top of file output
  section

Thanks to the user for catching this fundamental error.

Author: claude