Add GitHub Issues migration script (bd-68)

- New gh2jsonl.py script supports GitHub API and JSON file import
- Maps GitHub labels to bd priority/type/status
- Preserves metadata, assignees, timestamps, external refs
- Auto-detects cross-references and creates dependencies
- Production-ready: User-Agent, rate limit handling, UTF-8 support
- Comprehensive README with examples and troubleshooting
- Tested and reviewed

Amp-Thread-ID: https://ampcode.com/threads/T-2fc85f05-302b-4fc9-8cac-63ac0e03c9af
Co-authored-by: Amp <amp@ampcode.com>

examples/README.md

@@ -7,6 +7,7 @@ This directory contains examples of how to integrate bd with AI agents and workf
 - **[python-agent/](python-agent/)** - Simple Python agent that discovers ready work and completes tasks
 - **[bash-agent/](bash-agent/)** - Bash script showing the full agent workflow
 - **[markdown-to-jsonl/](markdown-to-jsonl/)** - Convert markdown planning docs to bd issues
+- **[github-import/](github-import/)** - Import issues from GitHub repositories
 - **[git-hooks/](git-hooks/)** - Pre-configured git hooks for automatic export/import
 - **[branch-merge/](branch-merge/)** - Branch merge workflow with collision resolution
 - **[claude-desktop-mcp/](claude-desktop-mcp/)** - MCP server for Claude Desktop integration