feat: update codex adapter and migrate to base acp prxy for agent (#1480)

TLDR:  upgraded codex acp bin and started moving over to similar proxy agent as claude agent sdk

## Problem

The Codex adapter was using a raw stream transformation approach that made it difficult to implement PostHog-specific features like session tracking, usage accumulation, and proper session resume/fork capabilities. This approach also lacked proper interception points for ACP protocol messages.

## Changes

**Upgraded codex-acp binary from v0.9.5 to v0.11.1** to get the latest features and bug fixes.

**Replaced raw stream transformation with proper ACP Agent implementation:**
- Created `CodexAcpAgent` class that implements the ACP Agent interface
- Added `CodexClient` to handle delegation between codex-acp subprocess and upstream PostHog Code client
- Implemented proper session state tracking with `CodexSessionState` and `CodexSettingsManager`
- Added support for session resume/fork operations (not natively supported by codex-acp)

**Enhanced PostHog integration:**
- Proper `_posthog/sdk_session` notification emission for session tracking
- Usage accumulation and `_posthog/usage_update` notifications
- Session metadata handling for task runs and persistence

**Improved architecture:**
- Moved from direct stream manipulation to proper ACP protocol handling
- Added consistent settings management interface shared with Claude adapter
- Better error handling and cleanup for subprocess management
- Proper terminal handle delegation for codex-acp terminal operations

Author: k11kirky

apps/code/scripts/download-binaries.mjs

@@ -19,7 +19,7 @@ const DEST_DIR = join(__dirname, "..", "resources", "codex-acp");
 const BINARIES = [
   {
     name: "codex-acp",
-    version: "0.9.5",
+    version: "0.11.1",
     getUrl: (version, target) => {
       const ext = target.includes("windows") ? "zip" : "tar.gz";
       return `https://github.com/zed-industries/codex-acp/releases/download/v${version}/codex-acp-${version}-${target}.${ext}`;