feat(cliproxy): add qwen code oauth provider support (#31)

Add 'qwen' as new CLIProxy OAuth provider with --qwen-login flag.
Support qwen3-coder model with base config at config/base-qwen.settings.json.
Register qwen OAuth endpoints and token prefix handling.
Update help documentation across all CLI entry points.

Implements issue #29

config/base-qwen.settings.json

@@ -0,0 +1,10 @@
+{
+  "env": {
+    "ANTHROPIC_BASE_URL": "http://127.0.0.1:8317/api/provider/qwen",
+    "ANTHROPIC_AUTH_TOKEN": "ccs-internal-managed",
+    "ANTHROPIC_MODEL": "qwen3-coder",
+    "ANTHROPIC_DEFAULT_OPUS_MODEL": "qwen3-coder",
+    "ANTHROPIC_DEFAULT_SONNET_MODEL": "qwen3-coder",
+    "ANTHROPIC_DEFAULT_HAIKU_MODEL": "qwen3-coder"
+  }
+}