docs: add alpha decision register and skills design

[JOY (JOY)]

Summary

• Add the alpha design decision register for settled alpha scope, assumptions, later JOY inputs, and out-of-scope guardrails.
• Add the alpha stats and skills design contract covering stat scale, archetype budgets, starter skill kits, semantic skill data, rank rules, AI intent use, UI requirements, and validation boundaries.
• Link the new docs from the GDD, vertical slice spec, systems index, roadmap, changelog, and GitBook summary.

Local review

• Local code-review fallback: APPROVED. Docs-only change; no backend/auth/social stack change; no Unity client model calls; no LLM authority expansion; server-owned validation boundaries preserved; cultivation/Nibirium remains deferred and out of alpha.

Verification

• npx.cmd markdownlint-cli2 "docs/design/12-game-design-document.md" "docs/design/14-character-stat-and-relationship-system.md" "docs/design/26-alpha-game-design-document.md" "docs/design/38-alpha-design-decision-register.md" "docs/design/39-alpha-stats-and-skills-design.md" "docs/design/03-systems-index.md" "docs/design/02-vertical-slice-spec.md" "docs/SUMMARY.md" "ROADMAP.md" "CHANGELOG.md"
• Markdown link resolver for changed docs: pass
• rg -n "—|–|…" ...changed docs...: no matches
• git diff --check
• git diff --cached --check