Add named parallel runtime instances

[wocha]

Summary

• adds --instance NAME and --instance=NAME support for named Codex runtime containers
• adds CODEXCLI_INSTANCE as an environment fallback
• keeps the default container name unchanged when no instance is configured
• validates instance names before Podman starts
• documents shared workspace and shared Codex home behavior
• expands --doctor output with base/effective container names and instance state

Validation

• bash -n scripts/*
• shellcheck -x scripts/*
• podman build -t codexcli-runtime-named-instances-final-smoke .
• manual --doctor checks for default, CLI instance, env instance, and CLI-over-env override
• invalid instance names fail before Podman
• git diff --check