feat: add MTPLX local MTP providers

[atomantic]

Summary

• Add disabled MTPLX API, OpenCode CLI, and OpenCode TUI provider presets for a separately managed local native-MTP runtime.
• Keep the existing Ollama Qwen3.8 GGUF path intact while dynamically configuring OpenCode's mtplx/ model namespace and on-demand model refresh.
• Add an additive migration, operator documentation, local-cost/quota safeguards, and provider parity coverage without installing or launching MTPLX.

Test plan

• NODE_ENV=test npx vitest run lib/opencodeConfig.test.js lib/providerModels.test.js lib/cliChildEnv.test.js services/agentCliSpawning.test.js services/agentProviderResolution.test.js lib/aiToolkit/validation.test.js lib/validation.test.js lib/aiToolkit/internal/modelFetchers.test.js lib/aiToolkit/defaults/providersSeedParity.test.js lib/aiToolkit/providers.test.js services/providerUsage.test.js services/quotaBurnJobs/providerPick.test.js lib/modelPricing.test.js ../scripts/migrations/272-mtplx-providers.test.js
• npm test -- --run src/utils/providers.test.js src/components/cos/TaskAddForm.test.jsx
• npm run lint
• npm run build