[2/3] core: track starting environments in snapshots

[sayan-oai]

Why

Remote environments may still be resolving when Codex creates a session or turn. Waiting for the existing all-or-nothing environment snapshot can hold startup until the selected environment is usable.

Behind the default-off deferred_executor feature, let callers take a useful snapshot immediately: completed environments remain available normally, while unfinished environments are reported without blocking startup. With the feature disabled, snapshots preserve the existing blocking behavior.

Depends on #28674.

What changed

• Store one ordered list of selected environments in ThreadEnvironments. Each selection owns one shared resolution that produces its complete TurnEnvironment.
• Start new resolutions in the background with remote_handle(), allowing snapshots and the future wait tool to share the same result while cancellation follows the retained handles.
• Make snapshot() a read-only operation: nonblocking snapshots collect completed resolutions and retain handles for unfinished ones, while blocking snapshots await every resolution.
• Replace completed failed resolutions from the current manager entry and log when failed environments are omitted.
• Return attached and starting environments as a point-in-time view, and count starting environments when deciding whether a snapshot is local-only.
• Keep existing consumers attached-only. to_selections() derives from attached environments, so child threads do not inherit an environment that is still starting.

Test plan

• just test -p codex-core environment_selection
• just test -p codex-core deferred_executor_reaches_model_before_remote_environment_is_ready

Landing note

Keep deferred_executor disabled for slow-starting executors until configurable environment/add connection timeouts and caller support land. When enabled, an environment that attaches after session startup may remain absent from environment-derived model context, tools, instructions, skills, and related state until follow-up refresh work lands.

[sayan-oai]

@codex review this

[chatgpt-codex-connector]

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[jif-oai]

I think the state machine we're trying to build here is not fully closed... this illustrate well my concerns on the difficulty to handle failure modes

[jif-oai]

Doesn't this make the remote’s AGENTS.md disappear for the whole loaded session, not just the turn that races startup?

[sayan-oai]

yep. the end state is going to be having these react to env readiness changes and get injected. that will be in a follow up, so i added a deferred_executor feature flag so we can land stuff incrementally without causing issues for existing testing.