feat: add attachment upload command

[hieptuanle]

Hi! I really like the project. But for now I cannot upload the images inside the markdown files as attachments. With this PR, I hope that this issue will be resolved.

Summary

• Add ol attachment create (alias ol att create) command for uploading file attachments to Outline documents
• Implement two-step presigned URL upload flow: POST metadata to attachments.create API, then upload file to the returned presigned URL
• Add apiUpload() function to the API layer for multipart file uploads
• Auto-detect content type from file extension (png, jpg, gif, svg, webp, pdf, zip)
• Update SKILL.md with attachment command documentation

Usage

# Upload an image to a document
ol att create ./screenshot.png --document <doc-id>

# With explicit content type and JSON output
ol att create ./file.pdf --document <doc-id> --content-type "application/pdf" --json

Test plan

• npm run build passes
• npm run lint:check passes
• npm run format:check passes
• npm test — 143 tests pass
• Manual test: uploaded image to Outline document, verified image renders correctly

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