Brainstorm-phase design for scaling Grogu from 1 to 3 self-hosted CI
runners. Covers components, precondition (PR #189 must land first),
sequential registration sequence, acceptance validation, failure
modes + mitigations, rollback, and explicit out-of-scope items.
Acceptance bar (minimum): green develop CI with >=3 SPM jobs in
flight simultaneously. Warm cache + runner-health monitoring are
explicit follow-ups.
Path follows the superpowers:brainstorming skill convention:
docs/superpowers/specs/YYYY-MM-DD-<topic>-design.md.
Refs #180#182#186#188