[ltmoerdani]

Summary

• Map the DeepSeek model family to the existing defer tool-image serialization strategy.
• Keep the tool message string-only and move image parts into a follow-up user message.
• Preserve existing MiMo drop, GLM 5.3 defer, and other-family multimodal behavior.
• Bump the extension to 0.7.12 and document the verified live Extension Development Host run.

Verification

• npm test: 570/570 passing
• npm run package: VSIX opencode-copilot-chat-0.7.12.vsix
• Staged lint: ESLint, Markdown, Editorconfig, TypeScript, and tests passing
• Live OpenCode Go / Deepseek V4 Pro verification in the existing novaliners Extension Development Host:
  browser screenshot capture completed, image attachment was produced, request finished with stop, and no HTTP 400 serialization error occurred.

Fixes #272