docs: use repo-relative local file links in README

[auyua9]

Summary

• replace the branch-pinned GitHub links for CONTRIBUTING.md and LICENSE in the root README with repo-relative links
• keep local repository file links branch-agnostic while preserving the existing external docs links

Testing

• not run (README-only change)

[cursor]

PR Summary

Low Risk
Low risk README-only change that just adjusts link targets; no runtime behavior or build output is affected.

Overview
Updates README.md to replace branch-pinned GitHub URLs with repo-relative links for CONTRIBUTING.md and LICENSE, keeping these references branch-agnostic and valid in local checkouts.

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