Clarify fork-based contributor workflow

[Parshan-MHB]

Summary:
Clarify that external contributors should use forks while maintainers use feature branches in the main repository.

What Changed:

• add an explicit access model section to CONTRIBUTING.md
• explain the practical GitHub enforcement boundary for forks versus maintainer branches
• keep the written workflow aligned with the current protected main policy

Verification:

• reviewed CONTRIBUTING.md against the current repository permissions and branch protection model

Security Impact:
Repository governance only. No runtime behavior change.

Screenshots:
N/A

[Parshan-MHB]

Superseded by #4, which recreates this change from current main and hardens the CI workflow to use an isolated scratch path with a bounded timeout.