Feat: Support global ~/.agents/AGENTS.md instructions

[nbiish]

Pi coding agent natively supports workspace-level AGENTS.md to guide project-specific behavior.

We should support loading a global instruction file from ~/.agents/AGENTS.md in the home directory. This allows the user's general developer rules (e.g. security protocols, coding style, reasoning loops) to guide the agent across all sessions without repeating settings.

[github-actions]

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See CONTRIBUTING.md.

[nbiish]

This setup will allow developers to establish a consistent, low-level global system prompt/ruleset across all agents on their machine, while still supporting project-level AGENTS.md/AGENT.md specifications for workspace-specific context.

[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.

[badlogic]

pi supports that.