Propagate exit code from REPL to process exit status

[cdprice02]

src/shell.rs lines 74-76 and 106 have TODO comments about setting exit codes in the caller environment rather than just breaking from the loop. Currently exit 0 and exit 1 both break the REPL but the exit code is discarded — ferrish's process always exits with status 0 regardless.

Expected behavior: Propagate the ExitCode from execute_command through run() and use std::process::exit() (or return it to main) so that the ferrish process reflects the requested exit status.

Acceptance criteria:

• exit 1 causes the ferrish process to exit with status 1
• exit 0 causes the ferrish process to exit with status 0
• The TODO comments in shell.rs are resolved
• This is critical for predictable semantics