Load configured environments from CODEX_HOME

[starr-openai]

Why

The earlier PRs add stdio transport support and the config-backed environment provider, but the feature remains inert until normal Codex entrypoints construct EnvironmentManager with enough context to discover CODEX_HOME/environments.toml. This final stack PR activates the provider while preserving the legacy CODEX_EXEC_SERVER_URL fallback when no environments file exists.

Stack position: this is PR 5 of 5. It is the product wiring PR that activates the configured environment provider added in PR 4.

What Changed

• Thread codex_home into EnvironmentManagerArgs.
• Change EnvironmentManager::new(...) to load the provider from CODEX_HOME.
• Preserve legacy behavior by falling back to DefaultEnvironmentProvider::from_env() when environments.toml is absent.
• Make environments.toml-backed managers start new threads with all configured environments, default first, while keeping the legacy env-var path single-default.
• Update the app-server, TUI, exec, MCP server, connector, prompt-debug, and thread-manager-sample callsites to pass codex_home and handle provider-loading errors.

Self-Review Notes

• The multi-environment startup path is intentionally tied to the environments.toml provider. Using >1 configured environment as the only signal would also expand the legacy CODEX_EXEC_SERVER_URL provider because it keeps local addressable alongside remote.
• The startup environment list is still derived inside EnvironmentManager; the provider only says whether its snapshot should start new threads with all configured environments.
• The thread-manager sample was updated to pass the current ThreadManager::new(...) installation id argument so the stack compiles under Bazel.

Stack

• 1. Add stdio exec-server listener #20663 - Add stdio exec-server listener
• 2. Add stdio exec-server client transport #20664 - Add stdio exec-server client transport
• 3. Make environment providers own default selection #20665 - Make environment providers own default selection
• 4. Add CODEX_HOME environments TOML provider #20666 - Add CODEX_HOME environments TOML provider
• 5. This PR: Load configured environments from CODEX_HOME #20667 - Load configured environments from CODEX_HOME

Split from original draft: #20508

Validation

• just fmt
• git diff --check
• bazel build --config=remote --strategy=remote --remote_download_toplevel //codex-rs/thread-manager-sample:codex-thread-manager-sample
• bazel test --config=remote --strategy=remote --remote_download_toplevel //codex-rs/exec-server:exec-server-unit-tests
• bazel test --config=remote --strategy=remote --remote_download_toplevel --test_sharding_strategy=disabled --test_arg=default_thread_environment_selections_use_manager_default_id //codex-rs/core:core-unit-tests
• bazel test --config=remote --strategy=remote --remote_download_toplevel --test_sharding_strategy=disabled --test_arg=start_thread_uses_all_default_environments_from_codex_home //codex-rs/core:core-unit-tests

Documentation

This activates CODEX_HOME/environments.toml; user-facing documentation should be added before this stack is treated as a documented public workflow.