fix(gh-search): Add smart defaults and fix duplicate results

[prosdev]

Problem

Two issues with GitHub search UX discovered during dogfooding:

1. No smart defaults: Users had to explicitly specify --state open to find open issues, making queries like "open issues that need implementation" return closed issues instead
2. Duplicate results: Same issue appeared 3-5 times in search results

Solution

Smart Defaults

• Default to --state open and --type issue
• 90% of searches want open issues, matching behavior of Linear/GitHub
• Users can still override with explicit flags

Deduplication

• Added explicit deduplication using Set to track seen document IDs
• Ensures each issue/PR appears exactly once in results

Changes

• CLI (gh.ts): Added default values to option definitions
• Indexer (indexer.ts): Added seenIds Set for deduplication
• Docs: Updated command description to document defaults

Testing

✅ Default search returns only open issues (no closed)
✅ No duplicate results (tested with various queries)
✅ Can override: --state closed returns closed issues
✅ Can override: --type pull_request searches PRs
✅ All pre-commit hooks pass

Examples

# Before: Had to specify --state open, got duplicates
dev gh search "adapter" --state open --limit 5
# → 5 results, all duplicates of same issue

# After: Smart defaults, no duplicates
dev gh search "adapter" --limit 5  
# → 1 result, open issue

# Can still search closed
dev gh search "subagent" --state closed
# → Returns closed issues