Antigravity flags requests whose system prompt identifies another vendor's
client and answers 429 Quota Exhausted. Move the existing Zed/Claude prompt
rewrite into a config-driven rule table and add case-preserving opencode ->
antigravity mapping.
Applied in the executor so only Antigravity requests are rewritten - the
shared openai-to-gemini translator also serves gemini, gemini-cli, vertex
and zed, which must not be touched.
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