Fix host-env get argument validation

[midwan]

Summary

• remove the value validation accidentally applied to host-env get
• add native coverage that invokes the real host-env CLI entry point
• record the correction in the v2.6 changelog

Root cause

The get branch accepts host-env get NAME, so argc is 3 and argv[3] is
the terminating null pointer. A value-validation check introduced during release
hardening treated that absent argument as invalid and returned before building
the host command.

Impact

host-env get NAME once again reads existing host variables. The regression
test mocks only the Amiberry traps and exercises the production argument
dispatch and command-building path, covering both get and set.

Validation

• confirmed the failure manually in Amiberry before the fix
• docker run --rm -v "$PWD":/work -w /work sacredbanana/amiga-compiler:m68k-amigaos make clean test package
• rebuilt and verified Host-Tools-2.6.lha
• git diff --check