DEA-4590: add Linear upload image downloads

[steve-wecompare]

Summary

• Adds an api_download MCP tool that downloads authenticated image responses through Shellgate and returns base64 with content type, filename, and byte length.
• Restricts downloads to image/png, image/jpeg, and image/webp, caps maxBytes at 20 MB, and returns clear errors for unauthorized, not_image, too_large, and download_failed.
• Redacts Authorization headers from gateway request logs and updates Shellgate agent instructions/docs for Linear upload image handling.

Linear

https://linear.app/deal-nl/issue/DEA-4590/shellgate-maak-linear-upload-images-downloadbaar-en-inspecteerbaar

Tests

• pnpm check green, existing Svelte warnings only.
• pnpm build green, existing Svelte warnings only.
• pnpm vitest run tests/integration/mcp.test.ts could not run in this environment because Testcontainers could not find a working container runtime.

Regression notes

• Queried DEA-4575 through the existing linear-api Shellgate target and confirmed the issue description contains a Linear upload URL.
• Confirmed comment 2eca7466-4796-48a3-850c-b2ba6758d19c contains text but no image URL.
• Live discover does not currently expose linear-uploads, so the real image download check can be completed after the target is configured and this MCP tool is deployed.