docs: add Garden personalization packet

[JOY (JOY)]

Summary

• Add docs/design/55-the-garden-personalization-and-memory-design.md for Garden visible memory, memorial, proof, notice wall, and NPC reaction rules.
• Link the packet from the alpha GDD, The Garden hub design, production backlog, decision register, GitBook summary, and roadmap.
• Create follow-up implementation issue Implement Garden visible memory and personalization slice #256 and link the roadmap item to it.

Verification

• npx.cmd --yes markdownlint-cli2 ROADMAP.md docs/SUMMARY.md docs/design/26-alpha-game-design-document.md docs/design/33-alpha-production-backlog.md docs/design/38-alpha-design-decision-register.md docs/design/51-the-garden-hub-design.md docs/design/55-the-garden-personalization-and-memory-design.md: pass, 0 errors.
• git diff --check: pass.
• rg -n "—|–" ...changed docs...: pass, no generated em dash/en dash.
• tools/windows/check-backlog-sync.ps1 -NoGitHub: WARN only for existing broad ROADMAP TODOs without nearby issue refs; tracked files check passed.

Local reviewer fallback

• Docs-only change; no Unity, Nakama, AGENTS.md, or .claude/CLAUDE.md changes.
• Explicitly forbids client-created durable memory, LLM-created durable state, paid decor, stat furniture, construction timers, and decor gacha in alpha.
• Follow-up implementation is tracked in Implement Garden visible memory and personalization slice #256.