[FEATURE] Set shell session context for skills or slash commands

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

If a skill or slash command ends up calling some script, it'd be nice for that script to be able to access the session info, like session ID.

You might enable this through something like an environment variable.

This is important context for any sort of integration, whether it's a fun creative use case or enterprise utility.

Proposed Solution

Environment variable that has the session ID which child processes can find.