docs: update authentication methods to reflect OAuth discontinuation (#3325)

* docs: update authentication methods to reflect OAuth discontinuation

Remove deprecated Qwen OAuth references and update documentation to
direct users to valid authentication methods (API Key, Coding Plan,
or Local Inference) following the OAuth free tier discontinuation on
2026-04-15.

Closes #3316

* docs: fix quickstart auth description to match actual /auth UI

The /auth command shows three options: Alibaba Cloud Coding Plan,
API Key, and Qwen OAuth (discontinued). Updated quickstart.md to
accurately reflect this UI instead of splitting into Option A/B/C.

Also updated settings.md, commands.md, and troubleshooting.md with
minor OAuth-related cleanups.

* docs: update .qwen workspace description in quickstart

Remove reference to 'Qwen account' since OAuth is discontinued.
The .qwen directory is created by Qwen Code itself for storing
credentials, configuration, and session data.

* docs: fix warning block formatting in quickstart

- Add missing '>' continuation for the OAuth discontinuation warning block

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

* docs: update README Qwen3.6-Plus description

- Remove mention of running Qwen3.6-Plus locally via Ollama/vLLM
- Keep only the Alibaba Cloud ModelStudio API key option

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

* docs: address review feedback - remove Local Inference from auth, add dual-region links

- Local Inference removed from auth method lists, kept as separate
  'Local Model Setup' section with detailed Ollama/vLLM config examples
- All links now provide dual-region URLs (Beijing + intl)
- .qwen workspace note restored to original meaning (cost tracking)
- Device auth flow error kept scoped to legacy OAuth
- API setup guide links updated with confirmed intl URL

---------

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

docs/users/quickstart.md

@@ -10,7 +10,7 @@ Make sure you have:
 
 - A **terminal** or command prompt open
 - A code project to work with
-- A [Qwen Code](https://chat.qwen.ai/auth?mode=register) account
+- An API key from Alibaba Cloud Model Studio ([Beijing](https://bailian.console.aliyun.com/) / [intl](https://modelstudio.console.alibabacloud.com/)), or an Alibaba Cloud Coding Plan ([Beijing](https://bailian.console.aliyun.com/cn-beijing/?tab=coding-plan#/efm/coding-plan-index) / [intl](https://modelstudio.console.alibabacloud.com/?tab=coding-plan#/efm/coding-plan-index)) subscription
 
 ## Step 1: Install Qwen Code
 
@@ -52,21 +52,26 @@ npm install -g @qwen-code/qwen-code@latest
 brew install qwen-code
 ```
 
-## Step 2: Log in to your account
+## Step 2: Set up authentication
 
-Qwen Code requires an account to use. When you start an interactive session with the `qwen` command, you'll be prompted to log in:
+When you start an interactive session with the `qwen` command, you'll be prompted to configure authentication:
 
 ```bash
-# You'll be prompted to log in on first use
+# You'll be prompted to set up authentication on first use
 qwen
 ```
 
 ```bash
-# Follow the prompts to log in with your account
+# Or run /auth anytime to change authentication method
 /auth
 ```
 
-Select `Qwen OAuth`, log in to your account and follow the prompts to confirm. Once logged in, your credentials are stored and you won't need to log in again.
+Choose your preferred authentication method:
+
+- **Alibaba Cloud Coding Plan**: Select `Alibaba Cloud Coding Plan` for a fixed monthly fee with diverse model options. See the [Coding Plan guide](https://bailian.console.aliyun.com/cn-beijing/?tab=coding-plan#/efm/coding-plan-index) ([intl](https://modelstudio.console.alibabacloud.com/?tab=coding-plan#/efm/coding-plan-index)) for setup instructions.
+- **API Key**: Select `API Key`, then enter your API key from Alibaba Cloud Model Studio ([Beijing](https://bailian.console.aliyun.com/) / [intl](https://modelstudio.console.alibabacloud.com/)). See the API setup guide ([Beijing](https://bailian.console.aliyun.com/cn-beijing/?tab=doc#/doc/?type=model&url=3023091) / [intl](https://modelstudio.console.alibabacloud.com/ap-southeast-1?tab=doc#/doc/?type=model&url=2974721)) for details.
+
+> ⚠️ **Note**: Qwen OAuth was discontinued on April 15, 2026. If you were previously using Qwen OAuth, please switch to one of the methods above.
 
 > [!note]
 >