docs: add multi-agent merge proposal for Mendix projects

Describes the problem of merging binary MPR files when multiple AI agents
work in parallel worktrees, evaluates solution options (MDL scripts, custom
merge driver, clean/smudge filters, SQLite changesets, locking), and
recommends a phased approach using git's custom merge/diff infrastructure.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako