feat: implement modular provider architecture with Codex CLI support

Implements a flexible provider pattern that supports both Claude Agent SDK
and OpenAI Codex CLI, enabling future expansion to other AI providers
(Cursor, OpenCode, etc.) with minimal changes.

## Architecture Changes

### New Provider System
- Created provider abstraction layer with BaseProvider interface
- Model-based routing: model prefix determines provider
  - `gpt-*`, `o*` → CodexProvider (subprocess CLI)
  - `claude-*`, `opus/sonnet/haiku` → ClaudeProvider (SDK)
- Providers implement common ExecuteOptions interface

### New Files Created
- `providers/types.ts` - Shared interfaces (ExecuteOptions, ProviderMessage, etc.)
- `providers/base-provider.ts` - Abstract base class
- `providers/claude-provider.ts` - Claude Agent SDK wrapper
- `providers/codex-provider.ts` - Codex CLI subprocess executor
- `providers/codex-cli-detector.ts` - Installation & auth detection
- `providers/codex-config-manager.ts` - TOML config management
- `providers/provider-factory.ts` - Model-based provider routing
- `lib/subprocess-manager.ts` - Reusable subprocess utilities

## Features Implemented

### Codex CLI Integration
- Spawns Codex CLI as subprocess with JSONL output
- Converts Codex events to Claude SDK-compatible format
- Supports both `codex login` and OPENAI_API_KEY auth methods
- Handles: reasoning, messages, commands, todos, file changes
- Extracts text from content blocks for non-vision CLI

### Conversation History
- Added conversationHistory support to ExecuteOptions
- ClaudeProvider: yields previous messages to SDK
- CodexProvider: prepends history as text context
- Follow-up prompts maintain full conversation context

### Image Upload Support
- Images embedded as base64 for vision models
- Image paths appended to prompt text for Read tool access
- Auto-mode: copies images to feature folder
- Follow-up: combines original + new images
- Updates feature.json with image metadata

### Session Model Persistence
- Added `model` field to Session and SessionMetadata
- Sessions remember model preference across interactions
- API endpoints accept model parameter
- Auto-mode respects feature's model setting

## Modified Files

### Services
- `agent-service.ts`:
  - Added conversation history building
  - Uses ProviderFactory instead of direct SDK calls
  - Appends image paths to prompts
  - Added model parameter and persistence

- `auto-mode-service.ts`:
  - Removed OpenAI model block restriction
  - Uses ProviderFactory for all models
  - Added image support in buildFeaturePrompt
  - Follow-up: loads context, copies images, updates feature.json
  - Returns to waiting_approval after follow-up

### Routes
- `agent.ts`: Added model parameter to /send endpoint
- `sessions.ts`: Added model field to create/update
- `models.ts`: Added Codex models (gpt-5.2, gpt-5.1-codex*)

### Configuration
- `.env.example`: Added OPENAI_API_KEY and CODEX_CLI_PATH
- `.gitignore`: Added provider-specific ignores

## Bug Fixes
- Fixed image path resolution (relative → absolute)
- Fixed Codex empty prompt when images attached
- Fixed follow-up status management (in_progress → waiting_approval)
- Fixed follow-up images not appearing in prompt text
- Removed OpenAI model restrictions in auto-mode

## Testing Notes
- Codex CLI authentication verified with both methods
- Image uploads work for both Claude (vision) and Codex (Read tool)
- Follow-up prompts maintain full context
- Conversation history persists across turns
- Model switching works per-session

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/server/src/routes/models.ts

@@ -3,6 +3,7 @@
  */
 
 import { Router, type Request, type Response } from "express";
+import { ProviderFactory } from "../providers/provider-factory.js";
 
 interface ModelDefinition {
   id: string;
@@ -93,7 +94,25 @@ export function createModelsRoutes(): Router {
         {
           id: "gpt-5.2",
           name: "GPT-5.2 (Codex)",
-          provider: "openai",
+          provider: "openai-codex",
+          contextWindow: 256000,
+          maxOutputTokens: 32768,
+          supportsVision: true,
+          supportsTools: true,
+        },
+        {
+          id: "gpt-5.1-codex-max",
+          name: "GPT-5.1 Codex Max",
+          provider: "openai-codex",
+          contextWindow: 256000,
+          maxOutputTokens: 32768,
+          supportsVision: true,
+          supportsTools: true,
+        },
+        {
+          id: "gpt-5.1-codex",
+          name: "GPT-5.1 Codex",
+          provider: "openai-codex",
           contextWindow: 256000,
           maxOutputTokens: 32768,
           supportsVision: true,
@@ -111,15 +130,25 @@ export function createModelsRoutes(): Router {
   // Check provider status
   router.get("/providers", async (_req: Request, res: Response) => {
     try {
-      const providers: Record<string, ProviderStatus> = {
+      // Get installation status from all providers
+      const statuses = await ProviderFactory.checkAllProviders();
+
+      const providers: Record<string, any> = {
         anthropic: {
-          available: !!process.env.ANTHROPIC_API_KEY,
-          hasApiKey: !!process.env.ANTHROPIC_API_KEY,
+          available: statuses.claude?.installed || false,
+          hasApiKey: !!process.env.ANTHROPIC_API_KEY || !!process.env.CLAUDE_CODE_OAUTH_TOKEN,
         },
         openai: {
           available: !!process.env.OPENAI_API_KEY,
           hasApiKey: !!process.env.OPENAI_API_KEY,
         },
+        "openai-codex": {
+          available: statuses.codex?.installed || false,
+          hasApiKey: !!process.env.OPENAI_API_KEY,
+          cliInstalled: statuses.codex?.installed,
+          cliVersion: statuses.codex?.version,
+          cliPath: statuses.codex?.path,
+        },
         google: {
           available: !!process.env.GOOGLE_API_KEY,
           hasApiKey: !!process.env.GOOGLE_API_KEY,