fix: handle registry rate limiting with retry and backoff

[breferrari]

Problem

The GitHub registry client in registry.rs treats HTTP 429 (rate limit) responses as fatal errors. Unauthenticated GitHub API requests are limited to 60/hour, and even authenticated requests are capped at 5,000/hour.

Impact

A user running multiple weave install or weave search commands in quick succession (or in CI) can hit rate limits and get opaque RegistryHttp { status: 429 } errors with no guidance.

Proposed solution

1. Parse Retry-After header on 429 responses
2. Implement exponential backoff with 2-3 retries
3. On persistent rate limit, show a clear message: "GitHub API rate limit reached. Run weave auth login for higher limits, or wait N seconds."
4. Consider adding If-None-Match / ETag caching to reduce request count