Feat: Support AGENTS.md and global ~/.agents/AGENTS.md standard

[nbiish]

Agent Zero uses profile-based prompts. We should add native support for the AGENTS.md standard (repository-level AGENTS.md and global ~/.agents/AGENTS.md). This would allow developers to seamlessly share their instructions across Agent Zero and other CLI coding tools.

[nbiish]

To clarify the motivation for standardizing on ~/.agents/AGENTS.md (the global .agents home directory file):

While several CLI tools have their own tool-specific global folders (e.g., ~/.claude/CLAUDE.md, ~/.config/kilo/AGENTS.md, ~/.deepagents/AGENTS.md), standardizing on a single, vendor-neutral ~/.agents/AGENTS.md file is crucial for modern multi-agent workflows.

When developers write custom skills, orchestrators, or MCP tools that trigger multiple CLI agents with different provider/model configurations, it is highly complex to keep multiple rules files synchronized. Using a unified ~/.agents/ directory in the home folder ensures that all agents and sub-agents can fetch their core guidelines (such as security standards, reasoning structures, and data handling rules) from a single source of truth, while still respecting project-level AGENTS.md specifications for workspace-specific context.