docs: add merge queue design document
Comprehensive design for the Beads-based merge queue:
- Merge-request schema (type, fields, ID convention)
- Queue ordering (dependencies, priority, age)
- Engineer processing loop with failure handling
- CLI commands (gt mq submit/list/status/integration)
- Integration branches for batch work
- HOP framing (validation layer, entity chains)
- Configuration options and observability
Closes gt-yls.
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