feat(keys): expand Grok client samples and tune free soft-gate default

Ship Use Key templates that match Grok Build / Codex best practice: env
vars + multi-model config.toml with api_backend=responses, env_key over
hardcoded secrets, and clearer shell/path guidance for Claude/Codex/OpenCode.

Also set free_quota_token_limit default to 500k (24h soft-gate), clean up
personal-dev-only comments, and keep billing test fixtures aligned.

frontend/src/components/common/__tests__/PlatformTypeBadge.grok.spec.ts

@@ -46,7 +46,7 @@ describe('PlatformTypeBadge Grok plans', () => {
     expect(wrapper.text()).toContain('SuperGrok Heavy')
     expect(wrapper.find('[data-testid="grok-plan-icon"]').exists()).toBe(true)
     expect(wrapper.find('[data-testid="grok-free-plan-icon"]').exists()).toBe(false)
-    // Heavy uses purple plan chip (personal-dev parity)
+    // Heavy uses purple plan chip
     expect(wrapper.html()).toContain('bg-purple-100')
 
     await wrapper.setProps({ platform: 'openai', planType: 'free' })