docs(benchmarks): fix measure-cli script path

[hy3560]

What changed

• Point the README and script usage examples to docs/benchmarks/performance/measure-cli.sh.
• Add a Go regression test that verifies the local script paths in both examples resolve.

Fixes #1446

How it was checked

• go build ./... — passed (Windows, Go 1.26.5).
• go test ./docs/benchmarks/performance -count=1 and go vet ./docs/benchmarks/performance — passed.
• Ran both measure-cli.sh --help examples from the repository root with Git Bash — usage printed.
• git diff --check — passed.

The broader Windows checks are not fully clean: go vet ./... and some manual/law test packages reference Unix-only syscall.Kill / unix.Flock; separate existing UI/law assertions also fail on this checkout. GNU make is not installed here, so I could not run the repository wrapper make pr-ready. These failures are outside the changed files; CI on Linux is still needed.

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[santoshkumarradha]

Hey, thanks for the PR. Could you sign the CLA when you get a chance so we can review it?

[hy3560]

Thanks for the reminder. I have already signed the CLA, but CLA Assistant is still showing it as pending on this PR. I’ll retry its recheck flow; if it continues to block review, please let me know what additional step is needed. I also added the suggested one-line Unreleased changelog entry in the latest commit.