Add bilingual SweetGold agent guide

[alanthssss]

What changed

• add an English repository-level AGENTS.md
• add a fully corresponding Simplified Chinese AGENTS.zh-CN.md
• link both documents with a bilingual navigation header
• capture the current SweetGold milestone state, maintenance scope, repository map, research-integrity rules, validation matrix, documentation expectations, and definition of done
• formalize the separation between development, release preparation, and an explicitly authorized formal release

Why

SweetGold has accumulated multiple research milestones, model releases, product workflows, hardware work, and UI tasks across many conversations. Future agents need one durable operating contract that distinguishes current facts from stale task history, preserves the project's experimental integrity, and keeps the user able to understand and take over ongoing work.

Developer impact

Agents working in this repository now have explicit instructions for:

• starting and scoping work safely
• preserving deterministic evaluation and untouched final seeds
• maintaining the dependency-free core and bilingual product surfaces
• verifying Python, optional ML, Arena, and launch-site changes
• preparing PRs and releases without exceeding user authorization

Validation

• git diff --cached --check
• verified matching section structure across the English and Chinese documents
• confirmed the commit contains only AGENTS.md and AGENTS.zh-CN.md

This is a documentation-only governance change; runtime and ML tests are not affected.