docs: refine skill origin model

[JOY (JOY)]

Summary

• Refine the alpha stats and skills design so physical stats are mostly body-bound while skills are origin-bound.
• Add the skill availability model: soul knowledge, body physical capability, body imprint, equipped gear/tool, soul-body sync, and server validation.
• Update the decision register, GDD links, systems index, roadmap, and changelog to reflect the skill origin model.

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 skill validation and carryover data boundaries are explicit.

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" "ROADMAP.md" "CHANGELOG.md"
• Markdown link resolver for changed docs: pass
• rg -n "—|–|…|body-bound skills|Skills belong to bodies|SoulRecordSkill|Layer \| Inputs" ...changed docs...: no matches
• git diff --check
• git diff --cached --check