Add collision resolution with automatic ID remapping

Implements --resolve-collisions flag for import command to safely handle ID
collisions during branch merges. When enabled, colliding issues are remapped
to new IDs and all text references and dependencies are automatically updated.

Also adds comprehensive tests, branch-merge example, and documentation.

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

Co-Authored-By: Claude <noreply@anthropic.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
 - **[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
 
 ## Quick Start
@@ -23,6 +24,10 @@ cd bash-agent
 # Install git hooks
 cd git-hooks
 ./install.sh
+
+# Try branch merge collision resolution
+cd branch-merge
+./demo.sh
 ```
 
 ## Creating Your Own Agent