Devcontainer: add the Codex VS Code extension, with AGENTS.md pointg to CLAUDE.md

.devcontainer/Dockerfile

@@ -10,7 +10,7 @@
 
 # -- toolchain: builds and checks the code -----------------------------------
 
 FROM mcr.microsoft.com/devcontainers/base:3.0.8-noble@sha256:d7c468679f45a52ad3673d06656b5bf16e488990b17216a1fa295d9e1f89d724 AS toolchain
 
 SHELL ["/bin/bash", "-euo", "pipefail", "-c"]
 
@@ -207,4 +207,4 @@
 # once it is built, and the one thing that does — the docker daemon started by
 # the docker-in-docker feature — asks for root through sudo itself.
 USER vscode
-RUN mkdir -p /home/vscode/.claude /home/vscode/.config/gcloud /home/vscode/.config/gh
+RUN mkdir -p /home/vscode/.claude /home/vscode/.codex /home/vscode/.config/gcloud /home/vscode/.config/gh

.devcontainer/devcontainer.json

@@ -31,14 +31,17 @@
     "mounts": [
         // Claude Code login and settings survive container rebuilds.
         "source=canvas-api-claude,target=/home/vscode/.claude,type=volume",
+        // Codex's ChatGPT sign-in, settings and chat history: its default home, kept the same way.
+        "source=canvas-api-codex,target=/home/vscode/.codex,type=volume",
         // The Google sign-in survives them too: the credentials Terraform reads live here.
         "source=canvas-api-gcloud,target=/home/vscode/.config/gcloud,type=volume",
         // And the GitHub one, so that `gh auth login` is answered once rather than once
         // per rebuild.
         "source=canvas-api-gh,target=/home/vscode/.config/gh,type=volume"
     ],
     "containerEnv": {
-        // Tells a Claude session it runs in here, the one place it may work.
+        // Tells an agent's session, Claude's or Codex's, that it runs in here, the one place
+        // it may work.
         "MATHTRAIL_DEVCONTAINER": "1",
         "CLAUDE_CONFIG_DIR": "/home/vscode/.claude",
         "DISABLE_AUTOUPDATER": "1"
@@ -57,6 +60,10 @@
             // Exact versions; the Claude Code extension matches CLAUDE_CODE_VERSION.
             "extensions": [
                 "anthropic.claude-code@2.1.280",
+                // OpenAI's coding agent, signed in with a ChatGPT plan. It reads AGENTS.md,
+                // which sends it to the same rules as Claude. The extension brings its own
+                // CLI and sandbox, so the image installs nothing for it.
+                "openai.chatgpt@26.1007.21434",
                 "golang.go@0.56.1",
                 "redhat.vscode-yaml@1.24.0",
                 "nefrob.vscode-just-syntax@0.10.2",

AGENTS.md

@@ -0,0 +1,9 @@
+# AGENTS.md
+
+The working rules of this repository are in [CLAUDE.md](CLAUDE.md), and they bind every coding agent, not only Claude Code. Read it in full before any work and follow it as written: what it asks of Claude, it asks of you. It stays the one copy of the rules; this file only says what stands in for the parts that are Claude Code's own:
+
+- **The cloud-session exception to "Devcontainer only"** is for a cloud session of Claude Code. A Codex session that does not see `MATHTRAIL_DEVCONTAINER=1` stops before its first edit, as that rule says.
+- **The review before a change is called done.** `/code-review high` is a Claude Code command, and you cannot run it; Codex's `/review` stands in for it, started by the author. Everything else in that rule holds as written: prepare the clean copy and run the checks in it, then stop and ask the author to run `/review` there; verify and handle its findings as the rule says, with at most the one second round it allows. Until a review has run, the report says that it did not.
+- **A live run's narrow permission.** Claude Code gets it in `.claude/settings.local.json`. Under Codex the author approves each of the run's commands once, as it comes, never for the rest of the session, so nothing approved outlives the run; do not ask for a looser sandbox or approval mode for it.
+- **A live run's report** names its roles as they were: what the author did and what Codex did.
+- **Memory.** Claude Code's memory is not shared with you. What has to outlive a session goes into the repository's documents, as CLAUDE.md says.

CLAUDE.md

@@ -100,7 +100,7 @@ Project context lives in these files, not in chat history.
   Each tool's exact version is pinned directly where it is installed — as an `ARG` in
   `.devcontainer/Dockerfile`, as a literal in `devcontainer.json`. No separate versions file,
   no checksum verification.
-- **Language.** Code, every comment in code and config files (Go, TS, SQL, protobuf, YAML, HCL, Dockerfile, justfile, `.env.example`, …), `CLAUDE.md`, `README.md`, `CONTRIBUTING.md` and `SECURITY.md`, the texts of the UI, the hints the child sees and the model's prompts are in English (R02).
+- **Language.** Code, every comment in code and config files (Go, TS, SQL, protobuf, YAML, HCL, Dockerfile, justfile, `.env.example`, …), `CLAUDE.md`, `AGENTS.md`, `README.md`, `CONTRIBUTING.md` and `SECURITY.md`, the texts of the UI, the hints the child sees and the model's prompts are in English (R02).
   - RUN.md and every document under `docs/` are in Russian (R25).
   - Reports to the author in chat are in Russian.
   - A comment, and any text a person reads from code or config — a description, an error message, a test's name — gives its reason in its own words. It cites no decision, task or section of the documents: no `(R58)`, `(T11)`, `(spec 15.3)`, `(RUN.md, rule 13)`. The documents do not outlive the code.
@@ -317,6 +317,7 @@ Inside the container:
   - To change a version: edit the literal everywhere it appears (Dockerfile, `devcontainer.json` for Docker or the Claude Code extension), rebuild the container. The Claude Code CLI and its editor extension are one version in two files, so `just claude-update` raises both at once — with a version, or to the newest published one — and prints what it changed. Terraform is pinned twice as well: `ARG TERRAFORM_VERSION` and `required_version` in `infra/terraform/versions.tf`, which refuses any other. Its providers and `.terraform.lock.hcl` are raised by hand too, `terraform providers lock -platform=linux_amd64 -platform=linux_arm64`, since Dependabot would lock them with a Terraform of its own (R56).
 - **Container marker.** `MATHTRAIL_DEVCONTAINER=1` is set only inside the container. A session that does not see it is on the host and must stop, unless it is a cloud session the author started (see "Devcontainer only").
 - **Claude Code.** Its config lives in the `canvas-api-claude` volume, so the login survives rebuilds. Auto-update is off; the version is pinned.
+- **Codex.** OpenAI's coding agent works here too, as a VS Code extension pinned in `devcontainer.json`, signed in with the author's ChatGPT plan; the version is pinned, so VS Code does not update it. The extension carries its own CLI and sandbox, which keep a command's writes to the workspace and the command off the network until the author approves more. Reading is not confined: like Claude, Codex can read the whole container, the credential volumes and a local `.env` included, and what it reads goes to OpenAI. Its home, `~/.codex`, lives in the `canvas-api-codex` volume. It reads `AGENTS.md`, which sends it to this file: the rules are kept once, here (R69).
 - **`gh`.** What a change looks like on github.com is otherwise invisible from in here, and two parts of it are asked for by name: the alerts of the code scan, which a task closes before the next one starts, and whether the delivery that carried the change ran. `gh auth login` is answered once: the credentials live in the `canvas-api-gh` volume, as gcloud's live in `canvas-api-gcloud`, where Terraform reads them.
 - **Ports.** 8080 (the API) and 5173 (Vite's dev server for the SPA) are forwarded to the host.
 - **Just recipes.** `just --list` shows all of them.

RUN.md

@@ -252,7 +252,7 @@
 
 ## Таблица 3. Вопросы автору без ответа
 
-Собраны 2026-10-14 из «Открыто» задач T00–T11. Открытые вопросы документов — Т- (spec 20), О- (product 15.2) и П- (privacy 12) — живут в своих разделах. Те, что касаются E2, решают его задачи: Т-1 — T61, Т-16 — T67, Т-5 — T76; остальные — при разбиении E15.
+Собраны 2026-10-14 из «Открыто» задач T00–T11. Открытые вопросы документов — Т- (spec 20), О- (product 15.2) и П- (privacy 12) — живут в своих разделах. Те, что касаются E2, решают его задачи: Т-1 — T61, Т-16 — T67, Т-5 — T76, П-11 — T72, до первого образца в `samples/`; остальные — при разбиении E15.
 
 | ID | Вопрос | Откуда | Предлагаю |
 |---|---|---|---|