#5 docs: add superuser runtime redesign tracker

Summary

Add the approved Superuser Runtime Redesign tracker and its 16 implementation tickets, mapped across the seven-PR stack. This documentation-only PR is stacked on PR #4 (codex/rootbackend-transport-execution-and-recovery).

.scratch/superuser-runtime-redesign/
├── spec.md
└── issues/  # PR1–PR7 implementation slices

Evidence

• Before: no feature tracker existed under .scratch/superuser-runtime-redesign/.
  After: one tracker spec and 16 issue files; git diff --cached --check passed and the scoped Cubic review found no issues.
• No product code changed; tests were not rerun for this documentation-only commit.

Merge Danger

Door: two-way

Reverting removes the tracker documents only.

Blast Radius: documentation

---

Summary by cubic

Adds the approved Superuser Runtime Redesign tracker with its spec and 16 implementation tickets, mapped across the planned seven-PR stack. This documentation-only commit adds .scratch/superuser-runtime-redesign/, introduces no product code changes, and passes git diff --cached --check; tests were not rerun because only docs changed. Merging is a two-way door — reverting simply removes the tracker documents.