Hide Skills UI: remove Settings Skills section + Usage skills widget

[yikkuro]

Summary

Step 3 of the hide Skills feature. This is a renderer-only UI removal (branched off main, independent of the earlier steps). It removes the three user-facing Skills management surfaces so users never see the "Skills" concept.

Removed

• Settings → Skills section in SettingsView.vue: the nav item + icon, the entire section template, skills from VALID_SECTIONS, the auto-refresh watcher branch, the onMounted loader, and all now-unused skill state/logic (built-in/custom lists, toggles, allowlist/managed-entry helpers, askAgent flow). Also dropped the now-unused router/useAgentStore.
• Usage panel skills widget in UsagePanel.vue: the section, chips, empty state, and the "view more skills" button, plus the loadSkills/goSkillSettings logic and skill CSS.
• i18n: the orphaned settings.* skills-management keys, settings.menu.skills, and usage.*Skills keys, removed from both en-US and zh-CN.

Intentionally kept (out of scope)

• The skill-integrity / tamper-detection dialog in App.vue and all integrity.* keys — it is a security gate.
• The window.openclaw.skills IPC surface (the integrity dialog still uses it).
• sidebar.skills (misnamed — it maps to "Channels").
• All backend / gateway / skill-scanning code.

Validation

• Renderer vue-tsc -b + vite build: clean
• Renderer vitest: 198/198 pass
• Main-process tsc --noEmit: clean
• Main-process vitest: all pass
• ESLint on changed files: 0 errors (only pre-existing warnings)