Port attachment management commands from jira-ticket-cli PR #60.

Files to create:

• tools/jtk/api/attachments.go - API client
• tools/jtk/internal/cmd/attachments/attachments.go - Commands

Commands:

• jtk attachments list <issue-key>
• jtk attachments add <issue-key> --file <path> (multiple --file supported)
• jtk attachments get <attachment-id> --output <dir>
• jtk attachments delete <attachment-id>

API functions:

• GetIssueAttachments, GetAttachment, AddAttachment, DeleteAttachment, DownloadAttachment
• FormatFileSize, FlexibleID helpers

Source: open-cli-collective/jira-ticket-cli#60