feat(enterprise): disable custom skill writes

[Gracker]

Summary

• Complete enterprise multi-tenant §0.5.6 custom skill v1 disposal.
• Disable custom skill write endpoints in enterprise mode with 404 + disabled_in_enterprise_mode.
• Keep validation/reload/read-only skill and vendor routes available.
• Load skills/custom/ in SkillRegistry so non-enterprise admin writes are readable after reload instead of becoming dangling files.
• Update the enterprise README checklist and §14.3 implementation notes.

Verification

• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npx jest --runInBand --forceExit src/routes/__tests__/skillAdminRoutes.test.ts src/services/skillEngine/__tests__/customSkillLoader.test.ts
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run typecheck
• git diff --check
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run validate:skills
• cd backend && PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run test:scene-trace-regression
• PATH="$HOME/.nvm/versions/node/v24.15.0/bin:$PATH" npm run verify:pr

[Gracker]

已通过 commit 67856ec (merge) + 023a84a (frontend rebuild) 合入 main，关闭此 PR。