docs: replace agent template with project guidance

[acedatacloud-dev]

Summary

• Replace the Mintlify starter AGENTS.md with AceDataCloud-specific content ownership and verification guidance.
• Distinguish generated OpenAPI/guides/MCP pages from hand-authored Docs pages using the current sync script.
• Keep one source of instructions for Codex and Claude Code through a thin CLAUDE.md import.

Verification

• Confirmed scripts/sync_from_platformbackend.py and .github/workflows/validate-sync.yml exist on the target branch.
• git diff --check passed. Documentation-only change; no publication sync was run.