Allow -before-NUMBER files in sponsors directory guard

[mattstratton]

Summary

• The sponsor-assets guard workflow blocks any PR touching static/img/sponsors/**.
• Adds an exception so files whose name contains -before-<number> (e.g. logo-before-2026.png) are allowed through without tripping the block or comment.
• If a change mixes exempt and non-exempt files, only the non-exempt files are listed in the block comment and the check still fails.

Test plan

• Open a test PR that only adds/modifies a *-before-2026.* file under static/img/sponsors/ and confirm the guard check passes with no comment.
• Open a test PR with a non-exempt file under static/img/sponsors/ and confirm the guard still comments and fails as before.

🤖 Generated with Claude Code

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[mattstratton]

Note that the proposed test plan in the PR description wouldn't actually work because you have to commit github actions to main before they run

[nimbinatus]

Thank you, @mattstratton !!