Remove Matthias Zepper from sponsor list

[Copilot]

Description

Removed Matthias Zepper from the Coffee Tier Heroes section in SPONSORS.md per sponsor's request. Replaced with placeholder text to maintain file structure.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring

Shimmy Philosophy Compliance

• Maintains lightweight binary size (≤5MB constitutional limit)
• Preserves zero-config principle
• Enhances OpenAI API compatibility
• Follows invisible infrastructure philosophy

Testing

• I have tested these changes locally
• I have run cargo test and all tests pass
• I have run cargo clippy with no warnings
• I have run cargo fmt

Legal Compliance

• All commits are signed off with DCO (git commit -s)
• I have the right to contribute this code under the project license
• If this includes third-party code, it's properly attributed and licensed

Binary Size Impact

Current binary size: N/A (documentation change)
New binary size: N/A
Change: ± 0 MB

Quality Considerations

• This change maintains backward compatibility
• Performance impact has been measured and documented
• Documentation has been updated where applicable
• Change aligns with roadmap priorities

Community Impact

• This change benefits the broader Shimmy community
• Breaking changes are clearly documented and justified
• Migration path is provided for breaking changes

Checklist

• My code follows the project's coding standards
• I have read the CONTRIBUTING.md guidelines
• I have added tests for new functionality (if applicable)
• I have updated documentation (if applicable)

Additional Notes

Single-line documentation change with no code impact. Verified no other references to the sponsor exist in the repository.

Original prompt

Remove Mathias Zepper from sponsor list

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