feat: Example Anthropic provider plugin + plugin-registered provider adapter

[TheArchitectit]

Summary

This is PR 3 of 3 in a stacked PR chain implementing the plugin-provider architecture for SmallCode.

This PR adds an example Anthropic provider plugin that demonstrates the full plugin-provider lifecycle, plus a generic adapter for plugin-registered providers.

Changes

Anthropic provider plugin (.smallcode/plugins/anthropic-provider/)

• plugin.json — manifest with tools, hooks, permissions, and provider declarations
• adapter.js — AnthropicProviderAdapter wraps the Anthropic API with SmallCode's IModelProvider interface
• pre-request.js — transforms SmallCode chat format to Anthropic API format (converts tool definitions, adds system messages)
• post-request.js — normalizes Anthropic responses back to SmallCode format
• on-error.js — handles Anthropic-specific errors (rate limits, auth failures)
• init.js — plugin initialization
• cleanup.js — plugin cleanup

Provider adapter (src/compiled/providers/openai_compat.ts)

• Added PluginProviderAdapter that wraps plugin-registered providers
• Handles the mismatch between plugin provider names and OpenAI model identifiers
• Falls back gracefully when a plugin provider isn't available

How it works

1. Plugin declares providers in its manifest
2. PluginLoader.loadAll() calls providerRegistry.register() for each declared provider
3. resolveProvider() in index.ts checks the registry before falling back to OpenAI-compat
4. The adapter handles format conversion between SmallCode's chat format and the provider's API

Why this PR exists

The Anthropic provider plugin serves as:

• A reference implementation for building custom provider plugins
• A working example of the manifest schema (tools, hooks, permissions, providers)
• A demonstration of the prompt-inject pattern for RAG/persona injection
• A test case for the plugin loader's provider registration flow

Plugin structure

.smallcode/plugins/anthropic-provider/
├── plugin.json       # manifest: tools, hooks, permissions, providers
├── adapter.js        # AnthropicProviderAdapter — IModelProvider implementation
├── pre-request.js    # Chat format → Anthropic API format
├── post-request.js   # Anthropic response → SmallCode format
├── on-error.js       # Rate limit / auth error handling
├── init.js           # Plugin initialization
└── cleanup.js        # Plugin cleanup

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Stacked with:

• PR 1: Plugin system core — ProviderRegistry, hooks, permissions
• PR 2: Built-in provider wizard — /provider command + .env/auth fixes

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