test: build-tag repo-only tests so consumer 'go test ./...' stays clean

[millerjp]

Summary

TestReadmeQuickStart_Compiles and TestLLMs_FullTxtIsUpToDate shell out to local paths (./scripts/gen-llms-full.sh) and assume the repo working tree. A consumer vendoring syncmap and running go test ./... hits non-skipped failures.

Scope

Move both tests behind //go:build repo_tools. Update the Makefile's test / check targets to pass -tags repo_tools for the in-repo suite.

Acceptance criteria

1. Consumer go test ./... on a vendored copy is clean.
2. Repo make check still exercises both tests.
3. makefile-targets-guard updated expected list if needed.

Source: security-reviewer agent.