Add first-class ChatGPT subscription provider support

demos/llm_routing/chatgpt_subscription/README.md

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+# ChatGPT Subscription Routing
+
+Route requests through your ChatGPT Plus/Pro subscription using Plano. Uses the OpenAI Responses API under the hood, targeting `chatgpt.com/backend-api/codex/responses`.
+
+## Setup
+
+### 1. Authenticate with ChatGPT
+
+```bash
+planoai chatgpt login
+```
+
+This opens a device code flow — visit the URL shown and enter the code. Tokens are saved to `~/.plano/chatgpt/auth.json`.
+
+### 2. Start Plano
+
+```bash
+planoai up config.yaml
+```
+
+### 3. Send a request
+
+```bash
+curl http://localhost:12000/v1/responses \
+  -H "Content-Type: application/json" \
+  -d '{
+    "model": "gpt-5.2",
+    "input": "Hello, what model are you?"
+  }'
+```
+
+Or use the test script:
+
+```bash
+bash test_chatgpt.sh
+```
+
+## How it works
+
+- `chatgpt/gpt-5.2` in the config tells Plano to use the ChatGPT subscription provider
+- Plano reads OAuth tokens from `~/.plano/chatgpt/auth.json` (auto-refreshes if expired)
+- Requests are proxied to `https://chatgpt.com/backend-api/codex/responses` with the required headers:
+  - `Authorization: Bearer <access_token>`
+  - `ChatGPT-Account-Id: <account_id>`
+  - `originator: codex_cli_rs`
+  - `session_id: <uuid>`
+
+## Available models
+
+```
+chatgpt/gpt-5.4
+chatgpt/gpt-5.3-codex
+chatgpt/gpt-5.2
+```
+
+## Managing credentials
+
+```bash
+planoai chatgpt status   # Check auth status
+planoai chatgpt logout   # Remove stored credentials
+```