Add Codex provider

[furrypinkrabbit]

Summary

Add a new codex provider backed by the local Codex CLI app-server protocol.

This lets agentix use Codex as an LLM provider while keeping agentix in control of tool execution, history reconstruction, and provider-state round-trip.

Changes

• Add Provider::Codex and Request::codex()
• Add codex feature and include it in cli
• Drive codex app-server --listen stdio:// for streaming output
• Map agentix tools to Codex dynamicTools
• Interrupt Codex turns on tool calls, execute tools locally, then resume via thread/inject_items
• Round-trip opaque OpenAI Responses items through provider_data["openai_responses_items"]
• Add Request::tool_choice(...) builder
• Add aaagw -i codex
• Add examples/13_codex_agent_multiturn.rs
• Bump crate version to 0.26.0
• Update README and CHANGELOG

Compatibility Notes

Codex app-server does not expose exact equivalents for every OpenAI/agentix option, so unsupported mappings now fail fast instead of silently degrading.

Unsupported for Provider::Codex:

• temperature
• max_tokens
• extra_body
• JSON object response mode
• JSON Schema with strict=false
• forced tool choice: required or named tool
• document/PDF input
• non-text tool results

Supported:

• streaming text
• image input
• dynamic function tools
• tool_choice=none
• ReasoningEffort
• strict JSON Schema via outputSchema
• opaque encrypted/provider state round-trip

Verification

• cargo check --features cli
• cargo test --lib --features codex
• cargo run --example 13_codex_agent_multiturn --features codex