#11151: feat: add DeepSeek provider support

## Summary Adds support for the DeepSeek provider (DeepSeek-V3, DeepSeek-R1). ## Changes - Adds `deepseek` provider configuration. - Adds `deepseek-api-key` auth choice. - Updates interactive onboarding to support DeepSeek. - Updates non-interactive onboarding to support `--deepseek-api-key`. - Adds unit tests for DeepSeek integration. ## Testing - Ran `pnpm test src/commands/onboard-auth.deepseek.test.ts` (Passed) - Ran `pnpm test src/commands/onboard-non-interactive.deepseek.test.ts` (Passed) - Verified non-interactive onboarding flow. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a new `deepseek` provider configuration (default + reasoning models), wires it into both interactive and non-interactive onboarding flows, and introduces tests to cover config application and non-interactive onboarding behavior. The changes follow existing onboarding patterns (provider config builder, `apply*Config` helpers, auth-choice routing, credential upsert) and extend the auth-choice set to include a DeepSeek API key option. <h3>Confidence Score: 3/5</h3> - This PR is close to mergeable but has a functional gap in the non-interactive CLI wiring for the new DeepSeek API key option. - Core provider/config changes are consistent with existing patterns and include tests, but the new `deepseekApiKey` option is not passed through by the CLI onboarding option mapping, so the advertised CLI flag won’t work in normal usage. - src/cli/program/register.onboard.ts (option passthrough for deepseekApiKey) <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->