[tests] Gate the live DeepSeek probe on claude-proxy changes.

[tests] Gate the live DeepSeek probe on claude-proxy changes.

Rename test-vps-live to test-claude-proxy-live to make its scope
clear, and make it a stamp-gated target: the probe runs only when
conf/common/claude-proxy.scm is newer than
.claude/.stamp-claude-proxy-live, so unchanged proxies skip the
live API call entirely.  No longer chained into test-vps-fresh —
the hermetic VM suite stays the deterministic gate.

Also sanitize the environment for every guix invocation: sourced
profile files put the system profile's guix modules ahead of the
current guix's on GUILE_LOAD_PATH, which makes guix fail with
record ABI mismatches when evaluating repository modules.  Strip
the GUILE_* variables via a single guix-env prefix and use
$(CURDIR) instead of backtick pwd throughout.

Co-Authored-By: Claude <noreply@anthropic.com>