Add spam prevention guideline to CONTRIBUTING.md

[Copilot]

Establishes explicit anti-spam policy in contribution guidelines.

Changes

• Added "Spam Prevention" section to CONTRIBUTING.md
• States maintainer right to remove spam, promotions, and off-topic content
• Clarifies prohibited behaviors: repeated identical comments, unsolicited promotions, off-topic discussions, unauthorized bots

Positioned before Community section for visibility.

Original prompt

Add contribution guideline against spam: be nice don't spam, the house keeps the right cleanup as needed.

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[Copilot]

Pull request overview

This PR adds a new "Spam Prevention" section to the CONTRIBUTING.md file to establish an explicit anti-spam policy for the project. The addition helps maintain the quality of discussions and contributions by clearly stating what behaviors are prohibited and that maintainers have the right to moderate content. The section is strategically positioned before the Community section to ensure contributors see these guidelines early.

Changes:

• Added a new "🚫 Spam Prevention" section with clear anti-spam guidelines and specific examples of prohibited behaviors

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