agents: add images and icon rules

[jasonyuezhang]

Add a basic set of rules for images and icons.

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Copied from getsentry#102534
Original PR: getsentry#102534

[propel-test-bot]

Update static/AGENTS.md with images & icons guidelines

Documentation-only PR that extends the frontend style guide. It adds concise rules on how icons and images must be stored, imported, and optimized, plus illustrative TypeScript/React snippets. No source code or build configuration is modified.

Key Changes

• Added two bullets under Design system → General practices clarifying icon (static/app/icons) and image (static/app/images) placement rules
• Inserted a new Images and Icons subsection (~50 lines) with do/don’t code examples covering SVG optimization, import aliases (sentry-images), and webpack-loader usage

Affected Areas

• static/AGENTS.md (documentation)

This summary was automatically generated by @propel-code-bot