[codex] Add agent collaboration tools and named destinations

[furukama]

Summary

• add list_agents and chat_with_agent so one agent can hand work off to another through the gateway
• route handoffs through named destinations with target-agent validation and reusable collaboration session keys
• cover the gateway path and container tool shims with focused collaboration tests

Validation

• biome check --write on the touched files
• npm_config_cache=/tmp/.npm-cache npx -y node@22 node_modules/typescript/bin/tsc --noEmit
• targeted collaboration/container vitest coverage passed earlier under the Node 22 shim; a later rerun from the detached worktree hit the known better-sqlite3 ABI mismatch because the shared install was built for the host Node 25 runtime