#8783: feat(provider/volcengine): support volcengine provider

Title Add Volcengine provider support + fix Feishu DM/group session key routing Summary - Add first‑class volcengine model provider (Ark base URL, API key auth, default model doubao-seed-1-8-251228). - Wire Volcengine into onboarding flows (CLI flags, auth choices, non‑interactive setup, preferred provider mapping). - Add implicit provider injection via VOLCENGINE_API_KEY + auth profiles. - Add provider usage stub + labels. - Add Volcengine docs and provider index entry. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds a first-class `volcengine` model provider (Ark OpenAI-compatible base URL, API key auth, and a default Doubao Seed 1.8 model), and wires it into onboarding flows (CLI flags, interactive and non-interactive auth choice handling, preferred-provider mapping, and implicit provider injection via `VOLCENGINE_API_KEY` / auth profiles). Also introduces a provider-usage stub/label entry and adds Volcengine provider documentation + index routing. This fits the existing provider architecture by extending the shared provider config builders (`src/agents/models-config.providers.ts`), auth-choice onboarding/apply paths (`src/commands/*`), and usage snapshot plumbing (`src/infra/provider-usage.*`). <h3>Confidence Score: 4/5</h3> - This PR is largely safe to merge, with a couple of small integration gaps and documentation drift to fix. - Provider wiring is consistent with existing patterns (Xiaomi/Moonshot), and changes are mostly additive. Main concerns are an out-of-sync provider list in `models status` and mismatched docs vs runtime defaults; both are straightforward to address and don’t indicate deeper architectural risk. - src/commands/models/list.status-command.ts, docs/providers/volcengine.md <!-- 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 -->