chore: import upstream snapshot with attribution

docs/doubao.md

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+---
+summary: "Doubao provider notes: API-key auth and Volcengine Ark request-limit tracking."
+read_when:
+  - Adding or modifying the Doubao provider
+  - Debugging Doubao API-key setup
+  - Explaining Doubao usage display
+---
+
+# Doubao Provider
+
+Doubao tracks Volcengine Ark request-limit headers by probing the chat-completions endpoint with a configured API key.
+
+## Setup
+1. Enable **Doubao** in Settings → Providers.
+2. Paste an API key in the provider settings, or set `ARK_API_KEY`, `VOLCENGINE_API_KEY`, or `DOUBAO_API_KEY`.
+3. Refresh provider usage.
+
+## Behavior
+- Endpoint: `POST https://ark.cn-beijing.volces.com/api/coding/v3/chat/completions`
+- Probe models: `doubao-seed-2.0-code`, `doubao-1.5-pro-32k`, `doubao-lite-32k`
+- Reads `x-ratelimit-remaining-requests`, `x-ratelimit-limit-requests`, and `x-ratelimit-reset-requests` when returned.
+- If the key is valid but rate-limit headers are missing, CodexBar shows the key as active and links to the dashboard for details.