docs: add issue assignment protocol and anti-sniping policy

[Kaap10]

Summary

Clarifies the issue assignment and PR submission workflow in CONTRIBUTING.md to prevent duplicated efforts, avoid PR sniping, and ensure a fair experience for all contributors.

Key Additions

• Finding & Claiming Issues: Explicitly requires contributors to comment and wait for official maintainer assignment before beginning work or raising a PR.
• One Issue at a Time: Established a 1-issue-per-contributor rule to give everyone in the community equal opportunity.
• Anti-Sniping Policy: Clarified that PRs opened for issues already assigned to active contributors will be closed without merge to respect the assignee's time.
• Inactivity Timeout: Added a 5–7 day stale assignment policy allowing maintainers to reassign inactive issues.
• Updated PR Steps: Aligned the Pull Request Process checklist with the assignment requirement.

Why

Recently, there was confusion where PRs were submitted and merged for issues already assigned to other active contributors. Having this policy explicitly documented provides clear expectations for contributors and gives maintainers a formal guideline to reference.