feat: add provider abstraction for review backends

TASKS.md

@@ -584,3 +584,110 @@ Configure project-local Claude Code discovery via `.claude/settings.local.json`
 - No secrets or absolute paths in any tracked file ✅
 
 Status: ✅ Complete
+
+---
+
+## Phase 6 — Provider Abstraction
+
+### Task 6.1 — Provider factory (`src/providers/factory.js`, `test/providers/factory.test.js`)
+
+Create `createChatProvider(config)` factory that validates and returns configured chat provider.
+
+**Requirements:**
+- Whitelist validates against `"openai"` only (currently)
+- Defaults to `"openai"` for any falsy/unknown value (null, NaN, empty string, numeric)
+- Throws descriptive error on unrecognized provider names containing both the invalid name and supported providers
+- No global state — all logic in pure function
+
+**Tests:** ~30 tests covering default provider, CHATGPT_MCP_PROVIDER env var, whitelist enforcement, case sensitivity, edge cases (null, NaN, whitespace, JSON strings, mutation checks).
+
+Status: ✅ Complete
+
+### Task 6.2 — OpenAI provider adapter (`src/providers/openai.js`, `test/providers/openai.test.js`)
+
+Create `openaiProvider.send(input, config)` thin interface that wraps existing OpenAI modules.
+
+**Interface:**
+```js
+provider.send(input, config) → Promise<{ content: string }>
+```
+
+Internally calls:
+1. `createOpenAIClient(config)` — returns client with API key
+2. `sendOpenAIResponse(client, { input: [{ role: "system", content }], model, temperature, maxOutputTokens })`
+
+**Tests:** 27 tests covering parameter passing, call ordering, error propagation, idempotency, edge cases (empty input, unicode, long input, object input).
+
+Status: ✅ Complete
+
+---
+
+## Phase 7 — Integration and Test Updates
+
+### Task 7.1 — Update all tool handlers to use provider abstraction
+
+All five handler files updated to use `{ loadConfig, createProvider }` dependency injection instead of `{ loadConfig, createOpenAIClient, sendOpenAIResponse }`.
+
+| Handler | File | Change |
+|---------|------|--------|
+| handleAskChatGpt | `src/tools/ask-chatgpt.js` | deps.createProvider(config) + provider.send() |
+| handleReviewPlan | `src/tools/review-plan.js` | same |
+| handleReviewCode | `src/tools/review-code.js` | same |
+| handleDebugIssue | `src/tools/debug-issue.js` | same |
+| handleArchitectureReview | `src/tools/architecture-review.js` | same |
+
+Status: ✅ Complete
+
+### Task 7.2 — Update handler tests to use provider mock pattern
+
+All five handler test files rewritten with `{ loadConfig, createProvider }` mock pattern instead of `{ createOpenAIClient, sendOpenAIResponse }`.
+
+**Key changes in tests:**
+- `createProvider` mock returns `{ send: sendMock }` instead of direct client mocks
+- All inputs use baseInputSchema `{ question, context, ... }` fields consistently
+- 28 tests per handler (success path, validation failure, config failure, budget failure, provider creation/send failure, dependency order, warnings propagation, result shape, short-circuit behavior)
+
+| File | Tests | Status |
+|------|-------|--------|
+| test/tools/ask-chatgpt.test.js | 27 | ✅ |
+| test/tools/review-plan.test.js | 28 | ✅ |
+| test/tools/review-code.test.js | 28 | ✅ |
+| test/tools/debug-issue.test.js | 28 | ✅ |
+| test/tools/architecture-review.test.js | 28 | ✅ |
+
+Status: ✅ Complete
+
+### Task 7.3 — Config and provider test coverage
+
+Added tests for `CHATGPT_MCP_PROVIDER` env var in `test/config/env.test.js` and new provider-specific tests.
+
+- **env.test.js**: Added section testing chatgptMcpProvider defaults to `"openai"` and accepts any string value
+- **factory.test.js**: ~30 tests covering factory behavior
+- **openai.test.js**: 27 tests covering send delegation
+
+Status: ✅ Complete
+
+### Task 7.4 — Final integration verification
+
+- All 579 tests pass across 20 test files
+- No regressions in existing coverage
+- `npm start` → tools/list shows same 5 tools with unchanged schemas
+
+Status: ✅ Complete
+
+---
+
+## Completion Summary
+
+| Phase | Description | Status |
+|-------|-------------|--------|
+| 0 | Repository Setup | ✅ Complete |
+| 1 | Core Utilities | ✅ Complete |
+| 2 | OpenAI Integration | ✅ Complete |
+| 3 | Tool Inputs and Prompts | ✅ Complete |
+| 4 | Tool Handlers | ✅ Complete (5 handlers, 139 tests) |
+| 5 | MCP Server and Registration | ✅ Complete (5 tools registered) |
+| 6 | Provider Abstraction | ✅ Complete (factory + adapter) |
+| 7 | Integration and Tests | ✅ Complete (579 tests across 20 files) |
+
+**Total:** All planned MVP tasks complete. 579 passing tests, zero regressions, all docs updated.