docs: Document provider addition process and npm package architecture

[Copilot]

What does this PR do?

Documents how to add providers to OpenCode. Corrects misconception that providers must be on models.dev.

Key findings:

• Most providers use single PR - models.dev is optional metadata registry, not requirement
• API URLs come from npm packages, not OpenCode - OpenCode dynamically installs packages at runtime via BunProc.install()
• PRs are small because they only add integration code (auth, config), not API clients

Architecture:

// In OpenCode PR (integration only):
"sap-ai-core": async () => ({
  autoload: !!serviceKey,
  options: { serviceKey, deploymentId, resourceGroup },
})

// Runtime behavior:
await BunProc.install("@mymediset/sap-ai-provider", "latest")
const fn = mod[Object.keys(mod).find(k => k.startsWith("create"))]
const sdk = fn({ name: "sap-ai-core", ...options })
// npm package contains API URL, request/response logic

Real example: SAP AI Core PR #5023 - added provider with single PR, NOT on models.dev, supports 40+ models via @mymediset/sap-ai-provider package.

Files created:

1. ADDING_PROVIDERS_MODELS.md (616 lines)

   • Three approaches: OpenCode-only (common), models.dev + OpenCode (rare), config-only (personal)
   • npm package architecture explanation
   • Dynamic vs bundled provider loading
   • Real examples: SAP AI Core, openrouter, vercel

2. PROVIDER_ADDITION_SUMMARY.md (249 lines) - Quick reference with decision tree

3. Updated CONTRIBUTING.md - Links to new guide

How did you verify your code works?

• Analyzed packages/opencode/src/provider/provider.ts - traced provider loading (lines 1045-1072 show bundled vs dynamic installation)
• Verified SAP AI Core exists in codebase, NOT in models.dev repo
• Confirmed npm package architecture: packages provide API URLs, OpenCode provides integration
• Traced models.dev flow: fetched from https://models.dev/api.json, cached locally, optional for providers

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.