Decision — magic number = 3 parallel projects

Decision

The Split and Merge platform supports exactly 3 active projects in parallel. This is enforced as MAX_ACTIVE = 3 at the Monday webhook and as a hard constraint across orchestrator, agent-playbook deploy, and MC panel layout.

Rationale

Three projects is the proven supervisor-throughput ceiling for one human reviewer. Below 3 wastes orchestrator capacity; above 3 the QA queue overruns the supervisor. The number is empirical, not theoretical.

Scope

Do NOT parameterize MAX_ACTIVE. Do NOT add support for "configurable concurrency". Do NOT treat 3 as a default that callers can override. If a future deploy needs different concurrency, that is a platform-level refactor, not a config knob.

Reference

• Workspace CLAUDE.md hard constraint
• mission-control/lib/state-machine.ts MAX_ACTIVE