Inject executor-aware filesystem into app-server

[starr-openai]

Summary

• expose a narrow core helper for selecting executor-aware environment from config
• make app-server MessageProcessor construction async so fs/* can use that environment
• inject app-server fs/* from the selected executor-aware filesystem instead of Environment::default()

Validation

• in progress: focused codex-app-server fs test on isolated cargo target

Stack

• base PR: Load remote repo skills through session environment #15033

[github-actions]

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