Support AGENT_SKILLS_PATH Environment Variable for Additional Skills Loading

What variant of Codex are you using?

CLI

What feature would you like to see?

Add support for an AGENT_SKILLS_PATH environment variable in Codex. When this variable is set, Codex should load agent skills from this path in addition to the default skills location.
Motivation
This would allow users to define a custom location for additional agent skills, without replacing the default skill set. It enables more flexible project setups and shared skill libraries across different environments, while keeping the built-in skills intact.
Proposed Behavior

Codex continues to load skills from its current default location as usual.
If AGENT_SKILLS_PATH is set, Codex also loads skills from the specified path, supplementing the default set.
If the variable is not set, Codex behaves exactly as it does today with no changes.

export AGENT_SKILLS_PATH=/path/to/your/custom_skills/

codex should load skills from /path/to/your/custom_skills/.agents/skills directory.

Additional information

_No response_