feat: Add markdown-to-jsonl converter example [addresses #9]

Add lightweight example script for converting markdown planning docs
to bd JSONL format. This addresses #9 without adding complexity to
bd core.

Features:
- YAML frontmatter parsing (priority, type, assignee)
- Headings converted to issues
- Task lists extracted as sub-issues
- Dependency parsing (blocks: bd-10, etc.)
- Fully customizable by users

This demonstrates the "lightweight extension pattern" - keeping bd
core minimal while providing examples users can adapt for their needs.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/README.md

@@ -6,6 +6,7 @@ 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
 - **[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