Add OpenAI Codex prompt bundle support

[0xAWM]

Summary

This PR adds first-class OpenAI Codex-style prompt bundle support to the OpenAI runtime.

It moves the Codex prompt shape into editable bundled resources, wires those resources into OpenAI request assembly, and fixes project/global memory loading so Codex-style prompts receive the
expected AGENTS.md / CLAUDE.md context.

Changes

• Added Codex prompt resources:

  • resources/prompts/openai-codex-base.yaml
  • resources/prompts/openai-codex-developer.yaml
  • resources/prompts/openai-codex-contextual-user.yaml

• Added OpenAI Codex prompt context assembly:

  • Builds prompt bundles from current resources instead of hardcoded test text.
  • Separates base instructions from dynamic environment and project context.
  • Supports developer-message emission when the model/provider advertises support.
  • Falls back to folding developer/contextual text into instructions when unsupported.

• Updated OpenAI runtime paths to use the new prompt bundle plumbing across responses, streaming, and websocket flows.

• Updated provider metadata for OpenAI Codex compatibility options.

• Fixed global memory handling for Codex-style prompts:

  • Loads project AGENTS.md from repo root to cwd.
  • Includes global AGENTS.md from supported home config locations.
  • Preserves AGENTS.md priority over CLAUDE.md.
  • Falls back to project/global CLAUDE.md only when no AGENTS.md context exists.

• Added/updated specs under specs/puffer-core/ documenting the prompt bundle and compatibility behavior.

Compatibility Notes

• Anthropic prompt behavior is not changed.
• Non-Codex OpenAI prompt behavior is preserved.
• Codex-style OpenAI requests now keep dynamic context out of the static base prompt when the provider supports the richer message shape.
• Providers that do not support developer/contextual-user channels still receive equivalent prompt content through the fallback path.

Verification

Ran:

• cargo test -p puffer-core openai::prompt_context::tests
  • 16 passed
• cargo fmt --all --check
• cargo check -p puffer-core
• git diff --check

cargo check still reports existing warnings, but exits successfully.