#18 bug: panics on EPIPE when stdout consumer closes early

Evidence: "f0rge issue list | awk ..." where awk exited early -> process panicked: thread main panicked "failed printing to stdout: Broken pipe (os error 32)" with full stack trace. Violates: no panics on hostile input, one-line errors, exit codes 0/1/2 (exit was 101). All stdout users (println!/print!) must treat BrokenPipe as clean exit (write via writeln! and ignore EPIPE, like issue #13 fix but on the output side). Verify: "f0rge issue list | head -1" exits 0, no stderr, no panic; same for --json and every cmd.