Add share link create command

[AndreyVMarkelov]

Summary

• Add dbxcli share link create <path> to create or retrieve an existing shared link
• Idempotent: returns existing URL if link already exists (handles shared_link_already_exists error)
• Fallback uses ListSharedLinks(path, DirectOnly=true) to avoid returning parent-folder links
• URL-only stdout output for easy scripting (url=$(dbxcli share link create /file.txt))
• Uses Dropbox default settings — no visibility/password/expiration flags in this PR
• Preserves backward compatibility with existing share list link command

Closes #77, closes #113.

Test plan

• Validates exactly one path argument required
• Rejects root path
• Prints URL for newly created file and folder links
• Uses default settings (nil) for CreateSharedLinkWithSettings
• Returns existing URL from error metadata without calling ListSharedLinks
• Fallback prefers exact PathLower match
• Fallback follows pagination correctly
• Fallback errors on HasMore with empty cursor (pagination guard)
• Fallback errors when no direct link found
• Non-conflict API errors pass through unchanged
• Existing share list link command still resolves correctly
• golangci-lint clean, all tests pass