feat(userland): environment defaults in init and stlxterm

[FlareCoding]

Summary

• With envp plumbing in place, nothing seeded the initial environment, so getenv(TERM) still returned nothing everywhere.
• init sets the system-wide baseline (PATH=/bin:/usr/bin, HOME, SHELL, LANG) once at the top of the process tree; every descendant inherits it.
• stlxterm declares TERM=xterm before spawning its shell, since only the terminal emulator knows which escape dialect it implements. The raw console shell correctly gets no TERM.

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