feat: add optional You.com search integration

[mouse-value-add]

Summary

Add You.com web search as an optional literature source alongside existing alphaXiv, OpenAlex, and bioRxiv sources.

What Changed

• New literature source: --source youcom for web search capability
• API integration: Uses You.com search API with optional authentication via YDC_API_KEY
• Consistent interface: Follows existing CLI patterns: orx lit --source youcom <query>
• Uniform output: Web results mapped to LitHit format for consistent JSON output
• Graceful fallback: Uses keyless API when no API key provided

Why Useful

Research often requires context beyond traditional academic papers:

• Recent developments not yet indexed in academic databases
• Industry research, blog posts, and technical discussions
• Broader context around research topics
• Current community discussions in ML/AI

Usage Examples

# Search current ML research
orx lit "transformer attention mechanisms" --source youcom --limit 5

# Get JSON output for programmatic use  
orx lit "neural networks 2024" --source youcom --json

# View a specific web result
orx paper "https://example.com/article" --source youcom

Environment Setup

# Optional: Set API key for enhanced results
export YDC_API_KEY="your_api_key_here"

# Works without API key using keyless endpoint
orx lit "machine learning" --source youcom

Integration Quality

• ✅ Compiles and builds successfully
• ✅ Follows existing code patterns and style
• ✅ Maintains backward compatibility
• ✅ Handles errors gracefully (network, API failures)
• ✅ Optional integration (existing users unaffected)
• ✅ Tested with both authenticated and keyless API access

Technical Details

• Uses reqwest HTTP client (already in dependencies)
• Consistent error handling with existing sources
• URL-based IDs for web results (makes sense for web content)
• Falls back to keyless API when no authentication provided
• Web results don't have publication dates or citations (appropriately marked as None)

The integration is designed to complement, not replace, existing academic sources. It provides researchers with access to current web content while maintaining the same familiar interface.

Tracking

Related to issue: youdotcom-oss/integration-tracking#175

[mouse-value-add]

Resolved the conflict with the recent literature-command refactor and rebased the integration onto the new architecture (0fde75b):

• You.com is now a discover youcom subcommand, matching the Keyword/Embedding/Openalex/Biorxiv structure, instead of the old lit --source youcom flag that no longer exists.
• orx paper <url> still recognizes You.com results by URL.
• Dropped an accidentally-committed rustup installer script and local test notes that shouldn't have been in the diff.

Verified on the branch: cargo build, cargo test (690 passed, 0 failed), and a live smoke test of orx discover youcom "attention mechanisms in transformers" --limit 2 against the keyless endpoint, which returned real results mapped into the LitHit JSON shape.

The integration stays fully opt-in: it only runs when explicitly invoked as discover youcom, uses YDC_API_KEY bearer auth when set, and falls back to the keyless endpoint otherwise. Happy to adjust the subcommand naming or flag set if you'd prefer a different shape.