feat(search): add DeepSeek provider

[orga-agent]

Summary

• add an explicit DeepSeek native-search adapter behind Core’s unified search contract
• expose explicit provider selection in CLI, MCP, Pi, DSH, and the container HTTP service
• document configuration, DSH credentials, and the intentional no-fallback behavior

Verification

• pnpm exec vitest run packages/web-core/test/search.test.ts packages/dsh-web/test/artifact.test.ts
• pnpm typecheck
• pnpm format:check
• pnpm test
• pnpm build
• pnpm test:release
• pnpm test:pack