feat: add Volcengine provider with DeepSeek-V4-Pro/Flash support

Add a new Volcengine (Volcano Engine Ark) provider for accessing
DeepSeek-V4-Pro and DeepSeek-V4-Flash via the Volcengine Coding API.

Changes:
- Add ProviderKind::Volcengine to config crate with default base_url
  pointing to Volcengine Coding API (api/coding/v3)
- Add DeepSeek-V4-Pro and DeepSeek-V4-Flash models to the agent
  model registry under Volcengine provider
- Add ApiProvider::Volcengine to TUI with full picker/dropdown support
- Wire up CLI --provider, config get/set/unset, and secrets resolution
- Add environment variable aliases: VOLCENGINE_API_KEY, ARK_API_KEY
- Ignore local dev scripts (*.cmd, backup/)

.gitignore

@@ -39,6 +39,7 @@ dist/
 # Generated
 outputs/
 tmp/
+backup/
 
 # Reference papers / large research blobs (keep locally if needed, don't ship)
 docs/DeepSeek_V4.pdf
@@ -48,6 +49,7 @@ docs/*.pdf
 
 # Local dev scripts and temp files
 *.sh
+*.cmd
 !scripts/**
 !.github/scripts/**
 test.txt