[codex] Scope sessions to current user

[stefanriegel]

Changed

• Filter listed Claude sessions to the plugin server's configured/current OS user.
• Apply the same owner check before killing a Claude PID.
• Add a small assertion-based regression check for the user filter.

Why

The Sessions plugin scanned every visible /proc Claude process, so one OS user could see another user's Claude session in a shared host.

Checks

• npm run build
• node --check src/server.js
• node --check dist/server.js
• node test/session-filter.test.mjs