feat(userland): cleaned up the shell's Ctrl+C and signal-death reporting

[FlareCoding]

Summary

• A ^C at the shell prompt was silently swallowed, and every signal death printed "Terminated by signal", including routine SIGINT deaths the terminal's ^C echo already narrates.
• The line editor now handles ^C readline-style: repaint with the cursor parked at end of line, print ^C, discard the buffer, and start a fresh prompt.
• reap_status matches bash's quiet list: SIGINT and SIGPIPE deaths report nothing, SIGTERM gains the "Terminated" label, and crash labels plus 128+sig exit codes stay unchanged.

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