feat: add retry logic with exponential backoff and Retry-After support

[MChorfa]

Adds configurable retry mechanism for failed API requests. New --max-retries flag (default: 3). Respects Retry-After headers. Full test coverage.

[felixfbecker]

Thank you for your contribution! A --max-retries option sounds good to me, but the Go SDK already supports a MaxRetries option that respects Retry-After etc. The CLI shouldn't reimplement the retry logic on top.

[MChorfa]

Hi! I have updated the PR to use the SDK native option.WithMaxRetries() instead of the custom middleware.

Changes made:

• Removed internal/retry package entirely (370 lines deleted)
• Now using option.WithMaxRetries(int) from the SDK
• The SDK handles exponential backoff, jitter, and Retry-After headers automatically

The --max-retries flag is preserved with the same semantics:

• Default: 3 retries
• Range: 0-10 (0 disables retries)

Tests pass (internal packages). Ready for re-review!