#13295: feat: add Eternal AI provider integration

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a new “EternalAI” provider option across onboarding and implicit provider resolution, plus documentation. It introduces `src/agents/eternalai-models.ts` for model discovery (fetching `/models` from a configured base URL with a static catalog fallback), wires `ETERNALAI_API_KEY` into env resolution, adds interactive + non-interactive onboarding flows to store credentials under `eternalai:default`, and registers an EternalAI provider config (`baseUrl`, `api: openai-completions`, models list). Documentation is added under `docs/providers/eternalai.md` and linked from the provider index. <h3>Confidence Score: 3/5</h3> - This PR is close, but has a couple of concrete correctness issues that will confuse users and can break setup depending on which URL is actually valid. - Key wiring for auth choice and provider config looks consistent, but the EternalAI base URL is inconsistent between docs and runtime constants, and the configured default model alias does not match the actual default model ID, both of which are user-facing and should be corrected before merge. - src/agents/eternalai-models.ts, docs/providers/eternalai.md, src/commands/onboard-auth.config-core.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->