docs: add design session backlog handoff

[JOY (JOY)]

Summary

• Adds a design-session backlog handoff that separates docs/config-ready work from runtime/main-dev-owned work.
• Links the handoff from GitBook SUMMARY and the agent handoff checklist.
• Captures current stop lines for Add Gate outcome report ledgers for living roster consequence #137, Add AI NPC debug tools and Play Mode verification checklist #140, Implement offline-agent policy UI and return report #149, Add AI NPC believability smoke tests and evidence pack #251, Track Unity presentation and pipeline experiments #263, Track post-alpha and beta roadmap decomposition #265, Capture full alpha route evidence pack #287, Track scoped NPC voice sessions and playback implementation lane #288, and runtime-owned alpha issues.

Verification

• git diff --check
• generated wording scan for em dash, en dash, stale Relay Yard naming, and placeholder markers: 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 setup/navigation handoff docs. No Unity scene, C#, Nakama runtime, backend tests, gameplay logic, issue closure, or evidence-pass claim changed. The new page is explicitly not the queue source of truth and points agents back to GitHub issues/Project #5.

Closes no issue. This supports #260 and the active design/backlog workflow.

Refs #260

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