Terminate host-env get output cleanly

[midwan]

Summary

• terminate successful host-env get output with one newline
• apply the same behavior to POSIX shells and Windows PowerShell
• add exact-output and CLI-dispatch regression coverage
• update the v2.6 changelog

Why

Manual Amiberry release testing confirmed that variable retrieval works after
the argument-validation fix, but the Amiga Shell prompt was rendered directly
after the value. Values are restricted to one line, so presenting the result as
one complete output line is clearer and consistent with the other tools.

Impact

host-env get NAME now displays the value cleanly before the next Amiga Shell
prompt. Missing variables retain their existing failure behavior and exit code.

Validation

• completed the full Amiberry pre-release smoke-test checklist
• exact POSIX behavior test verifies value with spaces\n
• Windows builder test verifies Console.Out.WriteLine
• docker run --rm -v "$PWD":/work -w /work sacredbanana/amiga-compiler:m68k-amigaos make clean test package
• rebuilt and verified the 34-file Host-Tools-2.6.lha
• git diff --check