[1/3] core: add remote environment connection lifecycle

[sayan-oai]

Why

Remote environments can be registered before their exec-server is first used. Starting the connection at registration time uses that startup window, while sharing one startup result prevents background work and capability calls from opening competing connections.

Keep initial startup simple: each environment makes one connection attempt using its configured transport timeout. A failed initial attempt is final for that environment, while an environment that disconnects after connecting can still recover on a later operation.

What changed

• Start URL and Noise environments in the background when they are added to EnvironmentManager. Provider snapshots are fully validated before connection work begins.
• Share one initial connection attempt and its saved result across metadata, process, filesystem, and HTTP callers.
• Keep configured stdio environments lazy until first use so registration does not launch a process.
• Tie background startup work to the environment lifetime so replacing or dropping an environment cancels unfinished work.
• After an established client disconnects, share one fresh connection attempt across concurrent callers. A failed attempt fails the current operation without permanently preventing a later attempt.
• Store the shared lazy client directly on Environment and expose small methods for starting, observing, and awaiting startup.

Test plan

• just test -p codex-exec-server
• just test -p codex-app-server turn_start_resolves_sticky_thread_local_environment_and_turn_overrides

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 70317a41ac

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[sayan-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. More of your lovely PRs please.

Reviewed commit: ff3f3a668d

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[sayan-oai]

@codex review this

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 918b9bcbb5

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
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[sayan-oai]

@codex review this

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Bravo.

Reviewed commit: 116f0a66c9

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".