feat: add DeepSeek Harness model plugin

[frostming]

Summary

• add a bub-dsh run_model plugin backed by the official DeepSeek Harness Python SDK
• reuse Bub root model, provider, API, and token settings while mapping dsh to deepseek-official
• pass structured JSON prompts unchanged and expose structured Harness run errors through DshRunError
• reuse Harness runtimes and map Bub sessions to runtime-local ids to avoid persisted-log collisions
• document installation, configuration, runtime behavior, and restart limitations

Validation commands

uv lock --check
uv run --group test pytest packages/bub-dsh/tests
uv run ruff check packages/bub-dsh
uv run ruff format --check packages/bub-dsh

Known limitation

Restarting Bub creates new Harness session ids because the current SDK cannot restore a persisted session into a new runtime process.