* fix(proxy): do not override explicitly-set CI env var
pmg forces CI=true for non-interactive (non-PTY) proxy runs so package
managers behave non-interactively. This clobbered a CI value the user set
explicitly (e.g. CI=false on a build server), changing downstream tool
behavior unexpectedly.
Only inject CI=true when CI is not already present in the environment,
preserving the user's intent. mergeEnv override semantics are left intact
since other overrides (HTTP_PROXY, etc.) must clobber.
Fixes#335
* test(proxy): snapshot/restore CI env explicitly in override test
Address review feedback: make the unset-CI subtest's intent explicit by
snapshotting the original CI value, unsetting it for the test, and
restoring it in t.Cleanup instead of relying on t.Setenv cleanup.
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Co-authored-by: Claude <noreply@anthropic.com>
