v0.16.0 — model catalog + picker + 6 new providers

Pick your model, not just your provider — plus 6 new providers.

• Curated model catalog per API provider (every id verified against live provider docs): the /model picker is now two-level — backend, then model — with notes, deprecation warnings, and a ✓ on the current choice. Typed fast-paths: /model deepseek-v4-pro (model-only), /model deepseek:deepseek-v4-pro (both), any free-form id accepted everywhere.
• DeepSeek default → deepseek-v4-flash (deepseek-v4-pro selectable). deepseek-chat/deepseek-reasoner are deprecated upstream on 2026-07-24 and marked as such in the picker. OpenAI default gpt-4o → gpt-5.4.
• New providers (OpenAI-compatible, zero new deps): moonshot (Kimi k2.6), zai (GLM 5.1), qwen (DashScope; mainland endpoint via DASHSCOPE_API_BASE), groq, xai (Grok 4.3), gemini-api (Google's OpenAI-compat endpoint). All get the inline key-prompt on switch.
• cc-copilot backends --models lists the whole catalog; cc-copilot init gains a numbered model menu.
• Fix: a model picked for any provider no longer leaks into ollama's default via CC_COPILOT_MODEL.

16 new tests (385 total). Upgrade: uv tool upgrade cc-copilot / pipx upgrade cc-copilot.