## Why
The next PR in this stack introduces configured environments, where the
provider knows both which environments exist and which one should be
selected by default. The existing manager derived the default internally
by checking for the legacy `remote` and `local` ids, and it treated
"remote" as equivalent to "has a websocket URL." That does not work
cleanly for stdio-command remotes because they are remote environments
without an `exec_server_url`.
**Stack position:** this is PR 3 of 5. It is the environment-model
bridge between PR 2's transport enum and PR 4's TOML provider.
## What Changed
- Add `DefaultEnvironmentSelection` to the `EnvironmentProvider`
contract:
- `Derived` preserves the old `remote`-then-`local` fallback behavior.
- `Environment(id)` lets a provider explicitly select a configured
default.
- `Disabled` lets a provider intentionally expose no default
environment.
- Move the legacy `CODEX_EXEC_SERVER_URL=none` default-disabling
behavior into `DefaultEnvironmentProvider`.
- Make `EnvironmentManager` validate explicit provider defaults and
return an error if the selected id is missing.
- Track `remote_transport` separately from `exec_server_url` so
stdio-command environments are still recognized as remote.
- Add `Environment::remote_stdio_shell_command(...)` for the TOML
provider added in the next PR.
## Stack
- 1. https://github.com/openai/codex/pull/20663 - Add stdio exec-server
listener
- 2. https://github.com/openai/codex/pull/20664 - Add stdio exec-server
client transport
- **3. This PR:** https://github.com/openai/codex/pull/20665 - Make
environment providers own default selection
- 4. https://github.com/openai/codex/pull/20666 - Add CODEX_HOME
environments TOML provider
- 5. https://github.com/openai/codex/pull/20667 - Load configured
environments from CODEX_HOME
Split from original draft: https://github.com/openai/codex/pull/20508
## Validation
Not run locally; this was split out of the original draft stack.
---------
Co-authored-by: Codex <noreply@openai.com>
## Summary
- Change `EnvironmentProvider` to return concrete `Environment`
instances instead of `EnvironmentConfigurations`.
- Make `DefaultEnvironmentProvider` provide the provider-visible `local`
environment plus optional `remote` environment from
`CODEX_EXEC_SERVER_URL`.
- Keep `EnvironmentManager` as the concrete cache while exposing its own
explicit local environment for `local_environment()` fallback paths.
## Validation
- `just fmt`
- `git diff --check`
---------
Co-authored-by: Codex <noreply@openai.com>
