docs: document DeepSeek as a first-class provider

DeepSeek was registered as a provider (packages/providers/src/index.ts) and
wired into the CLI (--mode deepseek) and dashboard account form, but was
never mentioned in README.md or docs/providers.md and docs/cli.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>