Add upload command for sending images/files to Devin sessions

Summary

This PR adds a new upload CLI command that allows users to upload images and other files to existing Devin sessions. The implementation follows the existing code patterns and uses the Devin API's /v1/attachments endpoint.

Key changes:

• Added upload_file_to_attachments() function that uploads files to /v1/attachments endpoint using multipart/form-data
• Added upload CLI command that takes session_id and --image/-i option
• Command uploads the file, gets a URL, then sends a message to the session with the attachment formatted as ATTACHMENT:"{url}"
• Supports both json and table output formats
• Includes proper error handling for missing files and API errors

Usage:

# Upload an image to a session
devin-cli upload <session_id> --image path/to/image.png

# Upload with an additional message
devin-cli upload <session_id> --image screenshot.png --message "Here's the bug I found"

# Get JSON output
devin-cli upload <session_id> -i data.csv --output json

Review & Testing Checklist for Human

⚠️ Risk Level: Yellow - Code follows documented API patterns but hasn't been tested end-to-end with a real Devin session.

• Test with a real session: Run devin-cli upload <session_id> --image <test_image> with a valid session ID and verify the attachment appears correctly in the Devin session
• Verify attachment format: Confirm that the ATTACHMENT:"{url}" format is correctly recognized by Devin (based on API docs, but should be verified)
• Test error cases: Try with missing file, invalid session ID, and potentially large files to ensure proper error handling
• Check file limitations: Verify if there are any file size or type restrictions from the API that should be documented or enforced

Test Plan

1. Create a test Devin session: devin-cli create --prompt "Test session"
2. Create a simple test image: echo "test" > test.txt (or use any real image)
3. Upload to session: devin-cli upload <session_id> --image test.txt
4. Check the Devin session UI to confirm the attachment is visible and accessible
5. Test with --message option and --output json option

Notes

• Implementation based on Devin API documentation: https://docs.devin.ai/api-reference/attachments/upload-files-for-devin-to-work-with
• The --image flag accepts any file type (not just images), following the API's support for all file types
• Existing tests continue to pass (one pre-existing test failure in test_auth_test_command unrelated to these changes)
• Link to Devin run: https://app.devin.ai/sessions/2885524ed0b2469e99a9068670f1b38e
• Requested by: @parkerduff