docs: restore Garden roster gate prep brief

[JOY (JOY)]

Summary

• Restores the missing Build Garden roster and Gate prep UI #138 Garden roster and Gate prep builder brief as docs/design/130-alpha-garden-roster-gate-prep-builder-brief.md.
• Expands the mission-board-gate-prep evidence template with roster inspection, public/debug field separation, and route-node separation notes.
• Updates alpha handoff coverage and builder brief navigation to point at the restored Build Garden roster and Gate prep UI #138 brief.

Verification

• git diff --check
• rg -n "—|–|Relay Yard|Relay Yards|\*\*\*" docs/design/130-alpha-garden-roster-gate-prep-builder-brief.md docs/playtests/_templates/mission-board-gate-prep docs/setup/alpha-open-issue-handoff-coverage.md docs/setup/alpha-playable-loop-handoff-coverage.md docs/design/152-alpha-builder-brief-index.md - no matches
• powershell -NoProfile -ExecutionPolicy Bypass -File tools\windows\check-backlog-sync.ps1
• npx.cmd markdownlint-cli2 "docs/**/*.md" "README.md" "ROADMAP.md" "AGENTS.md" ".claude/CLAUDE.md"

Local Review Fallback

APPROVED, docs-only. Scope is limited to #138 Garden roster and Gate prep handoff/evidence docs. No Unity UI, scene, prefab, Nakama runtime, mission authority, reward, inventory, or LLM behavior changed. The update restores a missing source doc referenced by issue history and improves implementation evidence for roster inspection, debug separation, route-node separation, and server-owned attempt boundaries.

Closes no issue. Issue #138 remains open until Unity implementation and Play Mode evidence exist.

Refs #138