core: allow restricting subagent environments

[frantic-openai]

Summary

• Add optional environment_ids to v1 and v2 spawn_agent.
• Validate explicit child selections against ready parent-turn environments, preserve parent order, and support an empty selection.
• Filter fork-inherited selected capability roots to the same environment subset.
• Add schema, selection, capability-root, and remote/local integration coverage.

Why

A root thread with both local and remote environments currently passes every selected environment to each child. That means a child intended to work only on Windows can still explicitly target local.

This MVP lets the parent spawn a child with, for example, environment_ids: ["windows"]. While that child is loaded, tool calls naming an omitted environment are rejected.

Known limitation

This is a spawn-time, loaded-session restriction. Environment selections are not yet persisted for cold resume; a V2 residency reload or later cold resume reconstructs the thread from default environment selections, so include_local may reattach local. Persisting the subset across resume is a follow-up.

Validation

• just test -p codex-core spawn_agent_environment_ids (5 passed)
• just test -p codex-core spawn_agent_tool_v (2 passed)
• just test -p codex-core --lib spawn_agent_restricted_fork_filters_selected_capability_roots (1 passed)
• just test -p codex-core --test all spawned_agent_can_restrict_inherited_environments (1 passed locally; body is guarded when no remote fixture is available)
• just fix -p codex-core
• just fmt
• git diff --check

I also ran the full just test -p codex-core suite earlier: 2813 passed, 137 failed, and 18 skipped. The failures were unrelated local-host fixture failures such as missing codex-code-mode-host / test_stdio_server and sandbox/PTY environment failures.