feat: add issue link command to attach URLs to issues

[lucleray]

Adds linear issue link command that uses the attachmentLinkURL GraphQL mutation to attach a URL to an issue, replacing the need for raw GraphQL:

# Link a URL to a specific issue
linear issue link ENG-123 https://github.com/org/repo/pull/42

# Auto-detect issue from git branch
linear issue link https://github.com/org/repo/pull/42

# With custom title
linear issue link ENG-123 https://example.com --title "Design doc"