feat: add provider abstraction for review backends

ARCHITECTURE.md

@@ -425,33 +425,35 @@ Rules:
 
 ## 12. OpenAI Integration
 
-Use OpenAI as a second-opinion provider only.
+Use OpenAI as a second-opinion provider only. The integration lives behind the provider abstraction layer:
 
 ```text
-Provider: OpenAI
-API: Responses API
-Model: configurable
-Temperature: low
-Output: structured text
+src/providers/factory.js    — createChatProvider(config) validates and returns configured provider
+src/providers/openai.js      — openaiProvider.send(input, config) thin adapter
+src/openai/client.js         — createOpenAIClient(config) low-level client
+src/openai/responses.js      — sendOpenAIResponse(client, params) API call wrapper
 ```
 
-Environment variables:
+### Provider Selection
+
+Configured via `CHATGPT_MCP_PROVIDER` env var (defaults to `"openai"`):
 
 ```env
-OPENAI_API_KEY=
+CHATGPT_MCP_PROVIDER=openai   # current default; accepts any string but factory validates at runtime
+```
+
+The factory whitelists `"openai"`. Any unrecognized value throws at provider creation time (not config load time). Null/NaN/falsy values default to `"openai"` for safe fallback.
+
+### OpenAI-specific Environment Variables
+
+```env
+OPENAI_API_KEY=                 # required — no default, throws if missing
 OPENAI_MODEL=gpt-5.1
 OPENAI_TEMPERATURE=0.2
 OPENAI_MAX_OUTPUT_TOKENS=2000
 ```
 
-The OpenAI integration should be isolated behind:
-
-```text
-src/openai/client.js
-src/openai/responses.js
-```
-
-This makes it easier to add other providers later, including Ollama.
+The provider pattern makes it straightforward to add other providers (Ollama, Anthropic) — implement the `send(input, config)` interface and register it in the factory's whitelist.
 
 ---
 
@@ -805,7 +807,10 @@ Later:
 - Add Gitea pull request workflow.
 - Add safe project summary generation.
 - Add optional OpenHands integration.
-- Add provider abstraction for OpenAI/Ollama/local models.
+- Implement Ollama provider (factory already supports it).
+- Implement Anthropic provider.
+- Add streaming responses support.
+- Add cost tracking per tool call.
 
 ---