Implement version migration tool

[akonan]

Description

Create tool to migrate wiremd files between syntax versions.

Features

• Detect wiremd version from file
• Migrate to latest syntax
• Show what changed
• Batch migration for projects
• Preserve comments and formatting where possible

CLI Usage

wiremd migrate mockup-v0.1.md --to v0.2
wiremd migrate ./wireframes/ --to latest --backup

Priority

Medium

Labels

enhancement, migration, versioning