[codex] migrate environment context to world state

[pakrym-oai]

Why

This stacks on #29249 and makes the typed environment world-state section the active source of model-visible environment context. It removes the parallel legacy environment-context representation while preserving the existing single-environment prompt shape and keeping persisted/diffed state limited to cwd.

What changed

• Build EnvironmentsState directly from each turn environment, including its own shell, without a default-shell fallback or intermediate environment DTO.
• Use EnvironmentsState for initial context and between-turn environment updates.
• Preserve the legacy single-environment rendering exactly; render multiple environments in sorted order.
• Serialize and diff only cwd; continue rendering live-only shell, date, timezone, network, filesystem, and subagent values.
• Move the existing environment rendering support under world_state and migrate the focused rendering, diff, and JSON round-trip coverage.
• Replace the world-state test macro with explicit mock section types.

Test plan

• just test -p codex-core world_state
• just test -p codex-core build_settings_update_items
• just test -p codex-core build_initial_context
• just test -p codex-core contextual_user_message