[codex] Exit top-level loop on stdin EOF

[juangrukat]

Summary

• stop the top-level Shen loop when stdin reaches EOF
• keep existing error printing for non-EOF read/eval errors
• update the generated Go top-level runtime to match the Shen source change

Why

When Shen is run with piped stdin or redirected file input, EOF becomes error: empty stream. The top-level loop traps that error, prints it, and then recurses, which causes an endless stream of error: empty stream messages after the input file has finished.

This treats that EOF condition as a clean termination signal instead.

Validation

• go test ./...
• make shen
• printf '(+ 1 2)\n' | ./shen
• ./shen < /Users/kat/Desktop/a/tests.shen