test(rename): cover no-arg module-path derivation from the git remote

[devantler]

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What

Extends scripts/rename-placeholders.test.sh to exercise the script's no-argument path — deriving the Go module path from origin's GitHub remote.

Why

rename-placeholders.sh is the first thing a newcomer runs after Use this template, and running it with no argument is its documented default:

With no argument it derives the path from origin's GitHub remote.

That branch parses the remote URL — strip everything up to github.com, handle the scp : vs https / separator, drop the .git suffix — but every existing test case passes an explicit module path, so the derivation was unexercised. A regression there ships a wrongly-pathed scaffold to anyone who runs the default.

What the new cases pin

• https remote: https://github.com/example-owner/renamed-project.git → module github.com/example-owner/renamed-project (asserts the go.mod repoint and no lingering template path).
• scp-style remote: git@github.com:example-owner/renamed-project.git → the same derived path (proves the git@host:owner/repo.git form parses identically).

A small make_copy() helper builds each fresh throwaway repo (the existing copy is already renamed); the cleanup trap is extended to the two new copies.

Validation

• sh scripts/rename-placeholders.test.sh → PASS (all cases; go build/go test still green).
• Mutation-checked (non-vacuous): dropping the .git-suffix strip from the derivation makes go.mod carry …renamed-project.git and the https case fails as expected; restored.
• shellcheck -s sh clean. Test-only change — rename-placeholders.sh is untouched.

Sibling to devantler-tech/dotnet-template#248, which covers the same untested derive-from-remote branch (awk PascalCase) in the .NET template.