[FEATURE] Implement multi-repository search

[ascender1729]

Description

Allow users to search for issues across multiple repositories simultaneously.

Why Needed

• Developers contribute to multiple projects
• Compare issues across repos
• Find best contribution opportunities
• Broader search coverage

Implementation Details

• Multi-repo input (comma-separated or chips)
• Parallel API calls to multiple repos
• Unified results list with repo badges
• Sort by combined relevance
• Handle rate limiting across repos

Architecture

1. User enters multiple repos (e.g., facebook/react, vercel/next.js)
2. Parse and validate repo list
3. Fetch issues from each repo in parallel
4. Merge and sort results
5. Display with repo identification

Files to Create/Modify

• src/lib/multi-repo-search.ts
• src/components/shared/MultiRepoInput.svelte
• src/components/results/ResultsContainer.svelte
• Update IssueCard to show repo name

Acceptance Criteria

• Can search 2+ repos at once
• Results merged and sorted
• Each result shows source repo
• Performance acceptable (<10s for 3 repos)
• Rate limit handling

Time Estimates

• With AI: 220 minutes
• Without AI: 1440 minutes