Support configurable user-level discovery paths for agents, skills, hooks, and instructions

Copilot CLI currently discovers shared user-level artifacts from fixed locations such as:

%USERPROFILE%\.agents

This is useful as a cross-tool default, but there is no supported way to relocate that discovery root for managed or XDG-style profile layouts.

COPILOT_CUSTOM_INSTRUCTIONS_DIRS only adds instruction directories. It does not provide equivalent discovery overrides for:

• Agents
• Skills
• Hooks
• Shared manifests or related metadata

COPILOT_HOME is not an appropriate substitute because it relocates broader Copilot application state, including sessions, databases, packages, caches, and lock files.

Requested behavior

Provide one or more documented environment variables or configuration settings for additional user-level discovery roots.

For example:

COPILOT_AGENT_DIRS
COPILOT_SKILL_DIRS
COPILOT_HOOK_DIRS
COPILOT_CUSTOM_INSTRUCTIONS_DIRS

Alternatively, support a unified root:

COPILOT_USER_CONFIG_DIR=C:\Users\<user>\.config\agents

with documented subdirectories:

agents\
skills\
hooks\
instructions\

Compatibility

The existing %USERPROFILE%\.agents location should remain a default discovery path. Configured paths should augment or override it through clearly documented precedence rules.

This would preserve the .agents interoperability convention while allowing:

• Enterprise-managed profile layouts
• XDG-style configuration separation
• Portable development environments
• Machines where the profile root should remain free of tool-owned directories

Related issues

• #1954 — XDG_CONFIG_HOME ignored; no equivalent available
• #4204 — Add .agents discovery for instructions, agents, and hooks

This request is complementary to #4204: that issue proposes broader .agents discovery, while this issue requests a supported way to configure where that user-level discovery tree lives.

ソースファイルもテストも指定されていません。まず、%USERPROFILE%.agents と COPILOT_CUSTOM_INSTRUCTIONS_DIRS に対する既存のユーザーレベルの探索を特定し、その後、設定可能なルートと優先順位をどのように表現すべきかを判断してください。既存のデフォルトパスを維持しながら、文書化された設定が要求されたアーティファクト種別をサポートすれば完了です。