Make environment providers own default selection

[starr-openai]

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. 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. This PR: 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. Load configured environments from CODEX_HOME #20667 - Load configured environments from CODEX_HOME

Split from original draft: #20508

Validation

Not run locally; this was split out of the original draft stack.