[v3.7.0] hardening: race-flake fix, superpowers integration, golden-help regeneration

[Delqhi]

Goal

v3.7.0 is the next hardening release after v3.6.0. Focus on quality-of-life
fixes and the superpowers integration.

Scope

1. superpowers integration ([v3.7.0] feat: sin-code superpowers — obra/superpowers integration with supply-chain pinning #55) — see companion issue
2. race-flake fix in cmd/sin-code/internal ([v3.7.0] fix: pre-existing race-flake in cmd/sin-code/internal (4min timeout) #56) — 4-min timeout
3. sin-code config init/show/validate (closes feat: sin config — Unified configuration management #34) — interactive wizard + pretty-print + schema check
4. README propagation (closes docs: Update all README.md files across OpenSIN-Code repos to current state #39) — at least 4 priority repos
5. sin-code self-update e2e (closes feat: sin update — Self-update command (pipx + Go rebuild) #33) — first real goreleaser release (v3.6.0) + smoke-test the update flow
6. CHANGELOG entries for v3.6.0 + v3.7.0 (closes docs: CHANGELOG.md — v3.0.0/v3.1.0/v3.2.0 release notes #49)

Definition of done

• All 6 sub-items done
• go test ./... -race -count=3 green on 3 consecutive full runs
• v3.7.0 tagged, homebrew-sin formula updated

Cross-refs

• v3.6.0 commit 65fd9a2 (latest released)
• v3.7.0 branch feat/v3.7.0-superpowers-integration
• [v3.7.0] hardening: race-flake fixes, golden_help regeneration, sin config init/show/validate #54 (prior v3.7.0 metaissue — can be closed as duplicate or kept as summary)