fix(interpreter): exec < file redirects stdin for subsequent commands

[chaliy]

Summary

• When exec < file has no explicit fd, set pipeline_stdin to the file content so subsequent read/cat commands consume it
• Hoisted file-reading logic out of the fd check to share between named-fd and default-stdin paths

Test plan

• exec_stdin_redirect spec test — exec < file + read reads from file
• Smoke test via CLI confirms end-to-end behavior
• All existing exec and bash spec tests still pass

Closes #960