[Nelmad]

Fixes #161.

The first copyFrom already creates the destination, so the following os.Link fails with EEXIST and the test skips before reaching its assertion. Remove that copy before creating the hard link.

Also assert that the source and destination still refer to the same file after the second copyFrom. Checking only the returned binary name would still pass if the same-inode branch were removed and the binary were unnecessarily copied again.

Only the existing test changes; production code and the fallback for environments without hard-link support are unchanged.

Validation on macOS arm64, Go 1.26.10:

• Before: the target test skips with file exists; the same-inode block has coverage count 0. After: the test passes and the block has count 1.
• go vet ./... passes in packages/core; go test -count=1 -race ./install/ passes all 131 top-level tests without skips.
• In a temporary Go overlay, disabling the same-inode branch still passes with only the fixture fix, but fails with the added assertion.
• The changed file passes the repository's CI gofmt rule. The install tests also cross-compile for Windows amd64; they were not executed on Windows.
• ./scripts/sg-unit-check.sh stopped in guard/shield because the execution sandbox disallows localhost listeners. That package passes when rerun with localhost access and an isolated HOME. The full repository suite was not completed for this change.

Linux execution was not repeated locally because the Docker daemon was unavailable. No real-agent integration or installer scripts were run locally: this change only repairs a unit-test fixture, and the tests use temporary files and settings.

Hosted validation subsequently passed for this commit: CI, CodeQL, and Trivy. CI includes all nine Go module jobs, six cross-platform builds, and the integration checks.

[codeyevsky]

Test-only fix: removes the stale destination before the hard-link, asserts same-inode after copyFrom. Verified locally, production code unchanged.