feat: Parallel/concurrent collection

[Snider]

Summary

Add support for parallel downloads to speed up large collections.

Use Case

Collecting 50+ repos or crawling large websites is slow sequentially. Parallelism can dramatically improve speed.

Commands

# Parallel GitHub collection
borg collect github repos LetheanNetwork --parallel 5

# Parallel website crawl
borg collect website https://docs.example.com --parallel 3

# Global setting
borg config set parallelism 4

Implementation

• Worker pool pattern
• Per-domain rate limiting (even with parallelism)
• Progress bar showing all workers
• Graceful shutdown on interrupt

Options

Flag	Default	Description
--parallel N	1	Number of concurrent workers
--rate-limit	none	Max requests per second per domain

Considerations

• Respect robots.txt crawl-delay
• Don't hammer single domains
• Handle worker failures gracefully

Acceptance Criteria

• Worker pool implementation
• Per-domain rate limiting
• Progress reporting for parallel tasks
• Graceful error handling
• Memory-efficient for large queues

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.