feat: add minimal POSIX sh shell (/bin/sh)

feat: add minimal POSIX sh shell (/bin/sh)

- user/sh.c: minimal sh-compatible shell with:
  - Line reading from stdin
  - Argument parsing (whitespace-delimited)
  - PATH resolution (/bin/, /disk/bin/)
  - Input/output redirection (< and >)
  - Pipeline support (up to 4 stages with |)
  - Builtins: exit, echo
  - External commands via fork+execve+waitpid
- Makefile: added SH_ELF build rule and initrd packaging as /bin/sh
- cppcheck clean, 19/19 smoke tests pass