feat: Plexum-openai-provider v0.1 — OpenAI codex CLI as backend

Plexum ProviderPlugin that wraps the local `codex` CLI binary
(OpenAI Codex CLI ≥ 0.135). Same CLI-driven pattern as
Plexum-gemini-provider and Plexum-anthropic-provider's contractor.

internal/runner/ (~200 LOC):
- Per Plexum turn, fork:
    codex exec [resume <thread_id>] \
      --skip-git-repo-check \
      --dangerously-bypass-approvals-and-sandbox \
      --json \
      [--model <m>] \
      "<last user msg>"
  in agent workspace, with stdin = /dev/null (codex would otherwise
  block waiting for additional input).
- Stream-parses JSONL events:
    thread.started        → save thread_id to .plexum-codex-session
    item.completed        → if type==agent_message, emit text_delta
    turn.completed        → capture usage (input/output/cached/reasoning)
    error / turn.failed   → emit canonical EventError
- Other event types (turn.started, item.started, reasoning, etc.)
  logged at debug, don't produce user-facing events for v1

cmd/plexum-openai-provider-plugin/ implements ProviderPluginWithAgent
(needs AgentContext.Workspace as cwd).

Model surface:
  default: ["codex"]  (no --model flag → CLI default; only thing
                       ChatGPT-account installs support)
  override via config.supported_models + config.model_args for
  API-key installs that want gpt-5 / o3 / etc.

HostConfig (all optional):
  binary           default "codex" (operator should set abs path for
                   systemd PATH)
  extra_args       appended before the prompt
  supported_models override the advertised model list
  model_args       map plexum id → CLI --model value

No api_key field — codex CLI handles auth via ~/.codex/ state
(ChatGPT login OR OPENAI_API_KEY env).

End-to-end verified against local install (ChatGPT login):

1. CLI turn 1:        "OpenAI built me."
2. CLI turn 2 (resume): "I said: 'OpenAI built me.'" (multi-turn ✓)
3. Fabric channel e2e: alice → cx agent → codex CLI → outbound REST →
   seq=23: "Use list comprehensions for concise, readable filtering and
            transformation of iterables."

Known: when daemon runs under systemd PATH doesn't include nvm/local
bin dirs; operator must set absolute binary path in config.json.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -0,0 +1,84 @@
+# Plexum-openai-provider
+
+Plexum ProviderPlugin that wraps the local `codex` CLI binary (OpenAI
+Codex CLI ≥ 0.135). Same CLI-driven pattern as `Plexum-gemini-provider`
+and `Plexum-anthropic-provider`'s contractor mode.
+
+## Status
+
+**v0.1**: subprocess-per-turn, JSONL stream parsing, per-workspace
+session continuity via `codex exec resume <thread_id>`. Single model
+`codex` advertised by default; operator can override.
+
+## Auth
+
+No API key in this plugin's config. The `codex` CLI handles auth via
+`~/.codex/` (ChatGPT login OR `OPENAI_API_KEY`). Run `codex login`
+once (or set env) and this plugin just shells out.
+
+## Install
+
+```bash
+cd ~/Plexum-openai-provider
+./scripts/install.sh
+```
+
+Operator notes in `~/.plexum/plugins/plexum-openai-provider/config.json`:
+
+```json
+{
+  "binary": "/home/<you>/.local/bin/codex",
+  "extra_args": []
+}
+```
+
+(absolute path is required when the daemon runs under systemd —
+systemd doesn't inherit shell PATH).
+
+Then `.plugins.allow += ["plexum-openai-provider"]` and:
+
+```bash
+plexum agent-add --model codex my-agent
+systemctl --user restart plexum
+plexum say --agent-id my-agent --session-id $(plexum new-session --agent-id my-agent) "hello"
+```
+
+## How it works
+
+Per Plexum turn:
+
+```
+codex exec [resume <thread_id>] \
+  --skip-git-repo-check \
+  --dangerously-bypass-approvals-and-sandbox \
+  --json \
+  [--model <m>] \
+  "<last user message>"
+```
+
+cwd = `<agent workspace>`, stdin = `/dev/null` (codex would otherwise
+hang waiting for additional input). Codex emits JSONL:
+
+```jsonl
+{"type":"thread.started","thread_id":"..."}
+{"type":"turn.started"}
+{"type":"item.completed","item":{"id":"...","type":"agent_message","text":"..."}}
+{"type":"turn.completed","usage":{"input_tokens":...,...}}
+```
+
+`thread.started.thread_id` is captured into
+`<workspace>/.plexum-codex-session` so the next turn passes
+`exec resume <id>` instead of `exec`. `item.completed` of type
+`agent_message` becomes `text_delta`; `turn.completed.usage` populates
+the canonical Usage on `message_end`.
+
+## ChatGPT account vs API key
+
+ChatGPT-account installs only support the default model; explicit
+`--model gpt-5-codex` etc. returns `400 invalid_request_error`. With
+an API key install, operator can declare more model ids via
+`config.supported_models` + `config.model_args`.
+
+## License
+
+Same as Plexum.