Is your feature request related to a problem?
I can't add supporting documents (big markdown file) or screenshots to a ticket

Describe the solution you'd like
Add attachment upload feature to the CLI

Describe alternatives you've considered
Manually drag and drop on the Web UI but Claude Code can't do that

Example usage
Show how you'd use this feature:

jtk issues attach JTK-123 --file=something.md
jtk attachments add JTK-123 --file=screenshot.png

Maybe get, list, delete too? So you can say to an agent - fetch JTK-123 including attachments and plan the feature.

Additional context
API:

https://docs.atlassian.com/software/jira/docs/api/REST/9.17.0/#api/2/issue/{issueIdOrKey}/attachments
https://docs.atlassian.com/software/jira/docs/api/REST/9.17.0/#api/2/attachment