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