docs: add designDocs English translations and note to remove #6 from AGENTS.md for non-Chinese readers

[jamesjmartin]

Changes

Design Docs Translations

Created designDocs/en/ with English translations of all existing Chinese design documents:

• base_feature.plan.md — Plugin architecture, Quick Launch, terminal integration, and project structure
• diff_feature_plan.md — Diff workflow, signal separation, accept/reject logic, and data model
• opencode_api_notes.md — Server API study notes: sessions, diffs, SSE events
• testing_strategy.md — Test architecture, turn scenarios, and manual test cases

The original Chinese files remain untouched.

AGENTS.md Cleanup

Added a self-documenting note to section 6 (语言规则 / Language Rules) for non-Chinese readers to remove this section which will cause their LLM model to respond in Chinese.

Intent was to allow for easier understanding for contributors who can not read Chinese while maintaining existing preference.