test: make install test fixture hermetic

[leemeo3]

Summary

Removes the system-specific /usr/local/bin/guild literal from the scanForGuildEntry test fixture by using a temp-dir guild path instead. The parser coverage for a human-style guild: list entry stays the same without depending on a real install prefix.

Linked quest or issue

Closes #84

Test plan

• go test -count=1 ./internal/install
• PATH="$(go env GOPATH)/bin:$PATH" make check passes locally (fmt + vet + lint + sqlcheck + test-race)
• New behavior covered by TestScanForGuildEntry
• Manual CLI/MCP exercise not applicable; test-only fixture change

Notes for reviewers

make install-tools attempted to install the Makefile-pinned golangci-lint@v1.60.3, but that version failed to compile locally with Go 1.26.4 in golang.org/x/tools@v0.24.0. I installed the CI workflow's golangci-lint@v1.64.8 and put $(go env GOPATH)/bin on PATH for the successful make check run.