docs: Add AGENTS.md for AI coding agent guidelines

[jatinkrmalik]

Summary

Adds a comprehensive AGENTS.md file that provides context and guidelines for AI coding agents working in this repository.

Key Sections

• 🚨 Critical Rule: Git Worktrees — Always use worktrees in /tmp/vocamac-* for parallel unrelated tasks. Never mess with the core workspace.
• Repository Structure — Full directory layout with descriptions
• Build & Run — All build, test, and run commands
• Code Style — Swift conventions, architecture patterns, error handling, performance considerations
• Testing Requirements — What to test, what not to test, naming conventions
• Website — How the static site works, local testing, deployment
• Git Workflow — Branch naming, commit messages (Conventional Commits), PR conventions
• Dependencies — Key deps and version requirements
• macOS Pitfalls — MenuBarExtra quirks, code signing, permissions, caching issues

Why

This ensures any AI coding session has the full context needed to:

• Follow project conventions
• Write good, tested code
• Not corrupt the working directory when doing parallel work
• Avoid known macOS/SwiftUI pitfalls we've already solved