docs: add Atomic Chat local provider setup guide

[yanalialiuk]

Proposal

Document using Atomic Chat as a custom OpenAI-compatible provider in ~/.codex/config.toml.

Draft docs branch: https://github.com/yanalialiuk/codex/tree/docs/atomic-chat-integration

model_provider = "atomic-chat"
model = "<model-id-from-/v1/models>"

[model_providers.atomic-chat]
name = "Atomic Chat"
base_url = "http://127.0.0.1:1337/v1"
wire_api = "chat"
env_key = "ATOMIC_CHAT_API_KEY"

export ATOMIC_CHAT_API_KEY=atomic-chat-local

Compare / open PR: main...yanalialiuk:codex:docs/atomic-chat-integration

Works with Codex CLI and the desktop app (same config file).

[Nicolas0315]

I checked current main for where this should land. The local docs/ files were intentionally reduced to links to the hosted Codex documentation in #20896, and the repo AGENTS.md now says not to add general product/user docs under docs/ because the official Codex docs live elsewhere.

Because of that, I don't think a patch adding an Atomic Chat guide inside this repository would be accepted as-is. This likely needs to be added in the source for the hosted Codex docs instead. The proposed TOML still matches the existing custom OpenAI-compatible provider shape:

model_provider = "atomic-chat"
model = "<model-id-from-/v1/models>"

[model_providers.atomic-chat]
name = "Atomic Chat"
base_url = "http://127.0.0.1:1337/v1"
wire_api = "chat"
env_key = "ATOMIC_CHAT_API_KEY"

I am leaving this as a docs-routing finding rather than opening a repo PR that conflicts with the current documentation ownership rule.