feat: add provider abstraction for review backends

AGENT_HANDOFF.md

@@ -55,6 +55,58 @@ Total: 139 orchestration-only tests, all passing.
 
 **All five MCP tools registered:** ask_chatgpt, review_plan, review_code, debug_issue, architecture_review.
 
+## Completed (Phase 6) — Provider Abstraction ✅
+
+Decoupled tool handlers from OpenAI implementation via a provider abstraction layer:
+
+### src/providers/factory.js
+- `createChatProvider(config)` validates `chatgptMcpProvider` against whitelist
+- Defaults to `"openai"` for any falsy/unknown value (null, NaN, numeric)
+- Throws with descriptive message on unrecognized provider names
+
+### src/providers/openai.js
+- `openaiProvider.send(input, config)` — thin adapter wrapping OpenAI modules
+- Internally calls `createOpenAIClient(config)` then `sendOpenAIResponse(client, params)`
+- Params include: input as system messages array, model, temperature, maxOutputTokens from config
+
+### Handler changes (all 5)
+- Old interface: `{ loadConfig, createOpenAIClient, sendOpenAIResponse }`
+- New interface: `{ loadConfig, createProvider }`
+- Each handler calls `provider = deps.createProvider(config)` then `provider.send(budget.input, config)`
+- Handler logic unchanged — only dependency injection changed
+
+### CHATGPT_MCP_PROVIDER env var
+- Defaults to `"openai"` when not set
+- Accepts any string at loadConfig time; validation happens in factory at provider creation
+- Currently only `"openai"` is whitelisted; others throw at createChatProvider() time
+
+## Completed (Phase 7) — Tests ✅
+
+### Handler tests updated (5 files, all using `{ loadConfig, createProvider }` mock pattern)
+| 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 | ✅ |
+
+### New provider/config tests (3 files)
+| File | Tests | Status |
+|------|-------|--------|
+| test/providers/factory.test.js | ~30 | ✅ covers default, whitelist, edge cases |
+| test/providers/openai.test.js | 27 | ✅ covers all send delegation paths |
+| test/config/env.test.js | +4 (added section) | ✅ covers chatgptMcpProvider env var |
+
+### Final verification
+- All 579 tests pass across 20 test files (up from ~523)
+- `npm start` → tools/list shows same 5 tools, unchanged schemas
+- Zero regression in existing test coverage
+
+## Completed (Phase 5)
+
+**All five MCP tools registered:** ask_chatgpt, review_plan, review_code, debug_issue, architecture_review.
+
 ### Task 5.1 - MCP server skeleton ✅
 
 Minimal MCP stdio server in `src/server.js`. MCP initialize handshake succeeds. No tools registered yet.
@@ -65,7 +117,7 @@ Minimal MCP stdio server in `src/server.js`. MCP initialize handshake succeeds.
 
 **Registration details:**
 - Uses shared `baseInputSchema` (question required + context, constraints, expectedOutput, projectSummary, taskSummary, relevantFiles, logs optional).
-- All 3 external deps injected: `loadConfig`, `createOpenAIClient`, `sendOpenAIResponse`.
+- External deps injected via dependency injection: `{ loadConfig, createProvider }` — provider abstraction (Phase 6).
 - Returns structured MCP tool result: `{ content: [{ type: "text", text }], isError, warnings }`.
 
 **Smoke test results (all passing):**
@@ -126,7 +178,7 @@ Project-local Claude Code discovery configured:
 
 ## Next Pending
 
-N/A — all planned tasks complete.
+No pending tasks. MVP complete. Future work: additional providers (Ollama, Anthropic), streaming responses, cost tracking, Dockerfile, CI pipeline, safe project summary generation.
 
 ## General Rules