Expose CLAUDE_SESSION_ID environment variable

Summary

Expose the session identifier as an environment variable so Bash commands can create session-scoped resources without race conditions.

Use Case

When multiple Claude Code sessions work in the same repository, they need unique identifiers for:

• Temporary index files (for isolated git operations)
• Lock files
• Session-specific temp directories

Currently there's no reliable way to distinguish sessions from within Bash.

Current State

The session ID already exists—visible in /status output:

Session ID: 8a718ed2-2856-435b-bd9a-63c5b8291b42

Suggested Implementation

export CLAUDE_SESSION_ID=8a718ed2-2856-435b-bd9a-63c5b8291b42

Minimal change: export the existing ID where session environment is initialized.