refactor(cli): extract shared execx.SpawnDetached helper

Collapse the two near-identical detached-spawn platform trios (telemetry __send_analytics, trail __refresh_trail_enablement) into a single portable execx.SpawnDetached. execx already owns the sibling detachFromTTY process-attr logic, so both platforms reuse it; the Unix side now uses Setsid (full session detach) instead of Setpgid.

SpawnDetached is a no-op under in-process go test, so unit tests can never accidentally fork the test binary; call sites keep their spawn seams for assertions.

Also drop issue-number references from comments and test messages in favor of self-contained descriptions.