chore(qa): move Ethos design kit out of frontend/ into tracked design-system/

The kit (SKILL.md, ethos.tokens.css, ethos-icons.svg, .oxlintrc.json) lived
inside frontend/ — the exact dir freestyle runs scaffold into. vite refuses a
non-empty target, so every session improvised a stash-aside (frontend_backup/,
frontend_temp/) and left scratch dirs behind. Move the kit to a sibling
design-system/ so frontend/ starts clean, and deliver it via the L0 conventions
channel (project.toml) rather than seeding files in the scaffold target: copy
the token/icon assets in (don't read them), SKILL.md for rules only. Un-ignore
frontend/ so scaffolded output is tracked going forward.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HMbPmZZjcXhR2crU82zZ8S

design-system/.oxlintrc.json

@@ -0,0 +1,8 @@
+{
+  "$schema": "./node_modules/oxlint/configuration_schema.json",
+  "plugins": ["react", "typescript", "oxc"],
+  "rules": {
+    "react/rules-of-hooks": "error",
+    "react/only-export-components": ["warn", { "allowConstantExport": true }]
+  }
+}