feat: support AGENTS.md open standard as project instruction file
Add AGENTS.md (https://agents.md/) as a recognized project instruction file alongside .atomcode.md, ATOMCODE.md, CLAUDE.md and claude.md. Lookup order (first match wins): .atomcode.md > ATOMCODE.md > AGENTS.md > CLAUDE.md > claude.md. This allows projects that already have an AGENTS.md to be picked up automatically by AtomCode without creating a separate .atomcode.md, enabling interoperability with other AI coding agents that follow the AGENTS.md open standard. Closes: #208 Co-Authored-By: AtomCode (doubao-seed-2.0-lite) <noreply@atomgit.com>