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/markdown-to-jsonl/example-feature.md

@@ -0,0 +1,49 @@
+---
+priority: 1
+type: feature
+assignee: alice
+---
+
+# User Authentication System
+
+Implement a complete user authentication system with login, signup, and password recovery.
+
+This is a critical feature for the application. The authentication should be secure and follow best practices.
+
+**Dependencies:**
+- blocks: bd-5 (database schema must be ready first)
+
+## Login Flow
+
+Implement the login page with email/password authentication. Should support:
+- Email validation
+- Password hashing (bcrypt)
+- Session management
+- Remember me functionality
+
+## Signup Flow
+
+Create new user registration with validation:
+- Email uniqueness check
+- Password strength requirements
+- Email verification
+- Terms of service acceptance
+
+## Password Recovery
+
+Allow users to reset forgotten passwords:
+
+- [ ] Send recovery email
+- [ ] Generate secure reset tokens
+- [x] Create reset password form
+- [ ] Expire tokens after 24 hours
+
+## Session Management
+
+Handle user sessions securely:
+- JWT tokens
+- Refresh token rotation
+- Session timeout after 30 days
+- Logout functionality
+
+Related to bd-10 (API endpoints) and discovered-from: bd-2 (security audit).