feat(code): add GPT-6 Astra support (#6141)

Deep Agents Code now promotes GPT-6 Astra in the model selector.

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The upstream OpenAI model catalog provides the model profile, while
dcode’s curated recommendations control which frontier models are
promoted in `/model`. This adds only the curated GPT-6 Astra entry and
relies on the existing selector fallback behavior when local profile
data lags.

Made by [Open
SWE](https://openswe.vercel.app/agents/57467168-9c13-5531-a447-e1179610c2f4)

---------

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

Author: mdrxy

libs/code/deepagents_code/tui/widgets/model_selector.py

@@ -103,6 +103,7 @@
     "openai:gpt-5.6-luna": "GPT-5.6 Luna",
     "openai:gpt-5.6-sol": "GPT-5.6 Sol",
     "openai:gpt-5.6-terra": "GPT-5.6 Terra",
+    "openai:gpt-6-astra": "GPT-6 Astra",
     "openai_codex:gpt-5.6-luna": "GPT-5.6 Luna",
     "openai_codex:gpt-5.6-sol": "GPT-5.6 Sol",
     "openai_codex:gpt-5.6-terra": "GPT-5.6 Terra",