docs(agents): add root agent guide

[musnows]

Summary

• add a root AGENTS.md with repository-specific guidance for future agents
• document the current Electron, preload, renderer, and Kun runtime boundaries
• record provider, packaging, validation, and develop PR workflow notes

Why

• reopens the root agent guide work from the closed PR docs(agents): add repository workflow guide #227 with the current project knowledge consolidated
• gives future agents a repo-local starting point instead of rediscovering the same architecture and workflow details

Tests

• git diff --check
• git diff --cached --check

Supersedes #227.