docs(cli): clarify that memtrace install requires sudo when npm prefix is system-owned

[ege-arhan]

Problem

When users run memtrace install on systems where the global npm directory is root-owned (such as default Linux installs under /usr/lib/node_modules), the command fails with EACCES: permission denied.

Solution

Document in docs/cli-reference.md that memtrace install requires sudo if the npm prefix is system-owned, pointing users toward the right invocation without ambiguity.

Impact

Prevents confusion and unhandled permission errors during self-updates.

Evidence

Verified docs table formatting and accurate rendering against existing command options.

Fixes #72

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