docs: fix contributing setup and AGFS references

[rocke2020]

Summary

• Clarify that Go is only required for Go SDK development under sdk/go, not AGFS source builds.
• Update AGFS native rebuild wording to refer to AGFS/RAGFS and the Rust binding.
• Replace the nonexistent third_party/agfs/ project-structure entry with current native dependency/RAGFS paths.
• Use uvx pre-commit install for hook installation because pre-commit is not declared in the repo dev environment.

Verification

• uvx pre-commit run --files CONTRIBUTING.md (Ruff hooks skipped: no Python files to check)
• git diff --name-only upstream/main...HEAD -> CONTRIBUTING.md
• git rev-list --left-right --count upstream/main...HEAD -> 0 1
• rg -n "AGFS \(Go\)|third_party/agfs|pip install pre-commit" CONTRIBUTING.md || true -> no output