[test] make internal/install tests hermetic: remove /usr/local/bin/guild hard-coding

[kunallanjewar]

Summary

internal/install/mcp_install_test.go:678 hard-codes /usr/local/bin/guild as a test fixture string. This bakes a specific install location into the suite and can cause behavior to differ between contributor machines and CI runners.

Affected files

• internal/install/mcp_install_test.go (line 678)

Acceptance

• Test fixture uses a synthesized or temp-dir path instead of the hard-coded one.
• Test passes identically across macOS, Linux, and Windows regardless of system layout.
• go test ./internal/install/... clean.